# SIP early offer and delayed offer

Canonical: https://warmtransfer.net/knowledge/sip-early-delayed-offer

Last verified: 2026-09-25

In Session Initiation Protocol (SIP), an initial INVITE request may carry a Session Description Protocol (SDP) offer in its body, but RFC 3261 also permits an INVITE without an offer[^24]. When an initial INVITE omits the SDP offer, the message flow operates as a delayed offer, requiring the answering entity to supply the offer in a subsequent response[^25][^26].

## Offer and answer mechanics

Under RFC 3261, a User Agent Server (UAS) that receives an INVITE without an offer must generate an SDP offer in a reliable response[^26]. If the initial INVITE contains no offer, base RFC 3261 specifies that the offer is carried in the first reliable non-failure response from the UAS—the 2xx response—and the User Agent Client (UAC) must return the answer in the ACK[^25].

When RFC 3262 reliable provisional responses are in use and an initial INVITE contains no offer, the offer must appear in the reliable provisional response if that is the first reliable message sent to the UAC[^28]. A UAC that sent an offerless INVITE and receives the offer in a reliable provisional response answers inside the PRACK request[^27]. RFC 6337 similarly describes that for an INVITE without SDP, the UAS offer goes in the first reliable 1xx response (answered in PRACK) or, if none, in the final 2xx response, adding that the UAS ought not include SDP in other responses to that INVITE[^30]. For offerless subsequent or re-INVITE requests, RFC 6337 recommends that the offer include all codecs the UA is currently willing and able to use, rather than only those negotiated in earlier exchanges[^31]. However, RFC 6337 notes that this recommended behavior is not always achievable, citing certain interworking scenarios as an example[^29].

## Early media and carrier interactions

A Cisco CUCM-to-CUBE configuration example from 2014 notes that almost all service providers require Early Offer SIP calls[^48]. Cisco's CUCM-to-CUBE configuration example recommends sending Early Offer from CUCM to avoid early media cut-through issues[^47]. In delayed-offer call setups from CUCM, callers may fail to hear PSTN-generated ringback or announcements because no media path is established before the call is answered[^44]. Cisco documents SIP reliable provisional responses (PRACK/100rel) as an approach to establish early media on delayed-offer calls without utilizing extra media resources such as Media Termination Points (MTPs)[^45].

Carrier provider 46 Labs states that its PSTN carriers always send early-offer INVITEs by default[^3]. 46 Labs accepts delayed-offer INVITEs, delivering the SDP offer in the first reliable 18x response or the 200 OK[^2]. For 46 Labs' carriers to return a reliable 18x on a delayed-offer call, the incoming INVITE must include 100rel in either the Supported or Require header[^1]. 46 Labs notes that without a reliable 18x response carrying SDP, a delayed-offer INVITE cannot establish early media (such as network ringback), and it recommends that customers use early-offer INVITEs[^4][^5].

## Cisco Unified Communications Manager offer modes

In Unified CM 8.6(1), SIP trunks used Delayed Offer (INVITE without SDP) by default[^32]. Unified CM 12.5(1) documentation lists Delayed Offer, Early Offer, and Best Effort Early Offer as configurable SIP trunk features[^17].

* **Media Termination Point Required**: On a Unified CM 8.6(1) SIP trunk, this setting statically assigns an MTP to every outbound call, limits the trunk to voice calls, and supports only G.711 or G.729 codecs[^34].
* **Early Offer support for voice and video calls (insert MTP if needed)**: Unified CM 8.6(1) inserts an MTP only when the calling device cannot supply the media characteristics required to construct an Early Offer[^33]. MTPs are inserted for calls from older SCCP phones, as well as calls arriving on SIP Delayed Offer or H.323 Slow Start trunks that are routed to an Early Offer trunk[^35]. Conversely, no MTP is needed for calls originating from SIP phones or newer SCCP phones, or for calls arriving on SIP Early Offer trunks, H.323 Fast Start trunks, or MGCP trunks[^36].
* **Best Effort (no MTP inserted)**: For Cisco Webex Edge Audio integrations, Cisco instructs setting the Unified CM SIP Profile field "Early Offer support for voice and video calls" to "Best Effort (no MTP inserted)"[^18]. While Cisco recommends early offer on the Unified CM trunk used for Webex Edge Audio, Edge Audio does not require it and supports delayed offer as well[^19].
* **SIP Rel1XX Options**: In Unified CM, the SIP Profile contains a SIP Rel1XX Options setting that can be configured to send PRACK for 1xx messages[^46]. As documented in 2013, CUCM supported reliable provisional responses only as a UAC sending PRACK and could not itself send 180 or 183 responses with Require: 100rel as a UAS[^43].

## Cisco CUBE interworking and configuration

Cisco Unified Border Element (CUBE) includes a Delayed Offer to Early Offer (DO-EO) feature that converts an incoming delayed-offer INVITE into an early-offer INVITE on the outgoing leg[^6]. When `early-offer forced` is active, CUBE constructs the outgoing Early Offer SDP from its locally configured codec list rather than using the calling side's media capabilities[^8].

The `early-offer forced` feature can be configured globally under `voice service voip > sip` using the `early-offer forced` command[^10]. It can also be enabled on an individual dial peer with `voice-class sip early-offer forced`[^9]. Cisco's Webex Calling Local Gateway IOS XE registration-based trunk configuration includes `early-offer forced` under the global `voice service voip` configuration[^22]. Cisco explains this command as forcing the gateway to send SDP in the initial INVITE rather than waiting for the neighbouring peer[^23].

CUBE also provides reliable provisional response configuration: 100rel can be required globally using `rel1xx require 100rel` under `voice service voip > sip`, or per dial peer using `voice-class sip rel1xx require 100rel`, with the dial-peer setting taking precedence[^42].

* **Media flow-around**: To use DO-EO in media flow-around mode on CUBE, media flow-around must be configured separately before the DO-EO settings[^7]. CUBE does not support DO-EO with media flow-around for video calls[^15].
* **Mid-call renegotiation options**: The command option `early-offer forced renegotiate` triggers a mid-call delayed-offer re-INVITE only for specific codecs (aacld, h263, h263+, h264, mp4a), while `renegotiate` always triggers it for all codecs[^16].
* **Renegotiation conflicts**: CUBE DO-EO mid-call renegotiation fails if `midcall-signaling block` or `midcall-signaling passthru media-change` is configured[^12]. CUBE does not trigger DO-EO renegotiation if the initial call uses transcoding[^14].
* **IP/port changes**: CUBE cannot change the IP address or port in the response to a locally triggered DO-EO re-INVITE[^13].

## Microsoft Teams Direct Routing requirements

Microsoft Teams Direct Routing does not support Delayed Offer INVITEs (INVITE without SDP)[^40]. WarmTransfer's reading of the sources is that a Unified CM call that reaches Teams Direct Routing as a Delayed Offer will not be supported, so the SBC in front of Teams must either receive an Early Offer from Unified CM or convert Delayed Offer to Early Offer itself (for example with CUBE early-offer forced)[^21].

For inbound calls to Direct Routing, the Teams 'Call accepted' event is converted by the SIP proxy into a 200 response with SDP, and the SBC is expected to send and receive media to the candidates in that SDP[^38]. In Direct Routing non-bypass mode a Teams media answer is converted to a 183 Session Progress whose SDP is replaced by the Media Processor's SDP, and only 1 183 per session is sent[^37]. When a media-bypass Direct Routing call is transferred to a client that cannot use media bypass, Direct Routing starts an ICE restart by changing ice-pwd and ice-ufrag and offering new media candidates in a re-INVITE[^39].

Direct Routing may send SIP messages larger than 1,500 bytes, mainly because of SDP, and Microsoft recommends the SBC strip SDP values such as ICE candidates or unused codecs before forwarding to UDP trunks[^41].

## See also

* [Troubleshooting Teams Direct Routing](https://warmtransfer.net/knowledge/teams-direct-routing-troubleshooting)
* [CUBE troubleshooting commands](https://warmtransfer.net/knowledge/cube-troubleshooting-toolkit)
* [Troubleshooting a Webex Calling Local Gateway](https://warmtransfer.net/knowledge/webex-local-gateway-troubleshooting)
* [SDP offer answer and codec negotiation](https://warmtransfer.net/knowledge/sdp-offer-answer)
* [Early media and ringback behavior](https://warmtransfer.net/knowledge/early-media-ringback)

## Applicability

Applies to: IETF SIP, Microsoft Teams Phone Direct Routing, Cisco CUBE, Cisco Webex Calling Local Gateway (CUBE), Cisco Webex Edge Audio, Cisco Unified CM, and 46 Labs SIP carrier services. Deployments: on-premises, hybrid, multi-tenant, and any. Sources checked 2026-09-25. Unified CM SIP trunk offer modes are documented for releases 8.6(1) and 12.5(1)[^32][^17].

## What remains uncertain

The current default and value list of the SIP Profile field Early Offer support for voice and video calls in later Unified CM releases is not covered by the sources below. How Teams Direct Routing handles an offerless re-INVITE (such as during hold or transfer) from the SBC is not covered by the sources below. Which SIP response Direct Routing returns to an INVITE without SDP is not covered by the sources below. Offer-mode requirements in first-party interop guides from major carriers are not covered by the sources below. Cisco SRND text on Best Effort Early Offer behaviour (which sources yield DO vs EO) and the Cisco recommendation to use Best Effort on all trunks is not covered by the sources below. MTP provisioning guidance for Mandatory (insert MTP if needed) Early Offer trunks is not covered by the sources below. Bring-your-own-carrier offer-mode requirements for other third-party cloud telephony platforms are not covered by the sources below. Offer mode on the Webex Calling Local Gateway PSTN and Unified CM facing dial peers is not covered by the sources below.

## Sources

[^1]: For 46 Labs' carriers to send a reliable 18x on a delayed-offer call, the INVITE must include 100rel in the Supported or Require header. Source: [Early Offer versus Delayed Offer SIP Invites](https://support.46labs.com/support/solutions/articles/156000368953-early-offer-versus-delayed-offer-sip-invites), Article body, 100rel paragraph. Checked 2026-09-25.
[^2]: 46 Labs accepts delayed-offer INVITEs, with the SDP offer provided in the first reliable 18x response and/or the 200 OK. Source: [Early Offer versus Delayed Offer SIP Invites](https://support.46labs.com/support/solutions/articles/156000368953-early-offer-versus-delayed-offer-sip-invites), Article body, delayed offer paragraph. Checked 2026-09-25.
[^3]: 46 Labs states that its PSTN carriers by default always send early-offer INVITEs. Source: [Early Offer versus Delayed Offer SIP Invites](https://support.46labs.com/support/solutions/articles/156000368953-early-offer-versus-delayed-offer-sip-invites), Article body, paragraph on carrier behaviour. Checked 2026-09-25.
[^4]: 46 Labs states that without a reliable 18x response carrying SDP, a delayed-offer INVITE cannot establish an early media session (for example network ringback). Source: [Early Offer versus Delayed Offer SIP Invites](https://support.46labs.com/support/solutions/articles/156000368953-early-offer-versus-delayed-offer-sip-invites), Article body, early media paragraph. Checked 2026-09-25.
[^5]: 46 Labs recommends that customers use early-offer INVITEs. Source: [Early Offer versus Delayed Offer SIP Invites](https://support.46labs.com/support/solutions/articles/156000368953-early-offer-versus-delayed-offer-sip-invites), Article body, closing recommendation. Checked 2026-09-25.
[^6]: CUBE's Delayed Offer to Early Offer (DO-EO) feature converts a delayed-offer INVITE it receives into an early-offer INVITE on the outgoing leg. Source: [Cisco Unified Border Element Configuration Guide - Cisco IOS XE 17.6 Onwards - Delayed Offer to Early Offer](https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/voice/cube/ios-xe/config/ios-xe-book/m_do-to-eo.html), Delayed Offer to Early Offer (feature overview). Checked 2026-09-25.
[^7]: To use DO-EO in media flow-around mode on CUBE, media flow-around must be configured separately before the DO-EO settings. Source: [Cisco Unified Border Element Configuration Guide - Cisco IOS XE 17.6 Onwards - Delayed Offer to Early Offer](https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/voice/cube/ios-xe/config/ios-xe-book/m_do-to-eo.html), Configure Delayed Offer to Early Offer for media flow-around. Checked 2026-09-25.
[^8]: With early-offer forced, CUBE builds the outgoing Early Offer INVITE for an incoming Delayed Offer call from its locally configured codec list rather than from the calling side's media capabilities. Source: [Cisco Unified Border Element Configuration Guide - Cisco IOS XE 17.6 Onwards - Delayed Offer to Early Offer](https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/voice/cube/ios-xe/config/ios-xe-book/m_do-to-eo.html), Delayed Offer to Early Offer (feature information / configuration). Checked 2026-09-25.
[^9]: CUBE early-offer forced can be enabled per dial peer with the command voice-class sip early-offer forced. Source: [Cisco Unified Border Element Configuration Guide - Cisco IOS XE 17.6 Onwards - Delayed Offer to Early Offer](https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/voice/cube/ios-xe/config/ios-xe-book/m_do-to-eo.html), Configure Delayed Offer to Early Offer (dial-peer level). Checked 2026-09-25.
[^10]: CUBE early-offer forced can be enabled globally under voice service voip > sip with the command early-offer forced. Source: [Cisco Unified Border Element Configuration Guide - Cisco IOS XE 17.6 Onwards - Delayed Offer to Early Offer](https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/voice/cube/ios-xe/config/ios-xe-book/m_do-to-eo.html), Configure Delayed Offer to Early Offer (global level). Checked 2026-09-25.
[^11]: The CUBE guide describes CUBE sending its own local IP address in the initial Early Offer INVITE and then generating a locally triggered re-INVITE to convey the sender's SDP details once known. Source: [Cisco Unified Border Element Configuration Guide - Cisco IOS XE 17.6 Onwards - Delayed Offer to Early Offer](https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/voice/cube/ios-xe/config/ios-xe-book/m_do-to-eo.html), Delayed Offer to Early Offer (feature overview / media flow-around). Checked 2026-09-25.
[^12]: CUBE DO-EO mid-call renegotiation fails if midcall-signaling block or midcall-signaling passthru media-change is configured. Source: [Cisco Unified Border Element Configuration Guide - Cisco IOS XE 17.6 Onwards - Delayed Offer to Early Offer](https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/voice/cube/ios-xe/config/ios-xe-book/m_do-to-eo.html), Restrictions for Delayed Offer to Early Offer. Checked 2026-09-25.
[^13]: CUBE cannot change the IP address or port in the response to a locally triggered DO-EO re-INVITE. Source: [Cisco Unified Border Element Configuration Guide - Cisco IOS XE 17.6 Onwards - Delayed Offer to Early Offer](https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/voice/cube/ios-xe/config/ios-xe-book/m_do-to-eo.html), Restrictions for Delayed Offer to Early Offer. Checked 2026-09-25.
[^14]: CUBE does not trigger DO-EO renegotiation if the initial call uses transcoding. Source: [Cisco Unified Border Element Configuration Guide - Cisco IOS XE 17.6 Onwards - Delayed Offer to Early Offer](https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/voice/cube/ios-xe/config/ios-xe-book/m_do-to-eo.html), Restrictions for Delayed Offer to Early Offer. Checked 2026-09-25.
[^15]: CUBE does not support DO-EO with media flow-around for video calls. Source: [Cisco Unified Border Element Configuration Guide - Cisco IOS XE 17.6 Onwards - Delayed Offer to Early Offer](https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/voice/cube/ios-xe/config/ios-xe-book/m_do-to-eo.html), Restrictions for Delayed Offer to Early Offer. Checked 2026-09-25.
[^16]: The early-offer forced renegotiate option triggers a mid-call delayed-offer re-INVITE only for specific codecs (aacld, h263, h263+, h264, mp4a), while renegotiate always triggers it for all codecs. Source: [Cisco Unified Border Element Configuration Guide - Cisco IOS XE 17.6 Onwards - Delayed Offer to Early Offer](https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/voice/cube/ios-xe/config/ios-xe-book/m_do-to-eo.html), Delayed Offer to Early Offer > midcall renegotiation. Checked 2026-09-25.
[^17]: The Unified CM 12.5(1) System Configuration Guide lists Delayed Offer, Early Offer and Best Effort Early Offer among the features configurable on SIP trunks. Source: [System Configuration Guide for Cisco Unified Communications Manager, Release 12.5(1) - Configure SIP Trunks](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/admin/12_5_1/systemConfig/cucm_b_system-configuration-guide-1251/cucm_b_system-configuration-guide-1251_chapter_01110.html), Configure SIP Trunks > SIP trunk overview feature list. Checked 2026-09-25.
[^18]: The Edge Audio guide instructs setting the Unified CM SIP Profile field 'Early Offer support for voice and video calls' to 'Best Effort (no MTP inserted)'. Source: [Cisco Webex Edge Audio Customer Configuration Guide](https://help.webex.com/en-us/article/xmsy7d/Cisco-Webex-Edge-Audio-Customer-Configuration-Guide), (Optional) Set Early Offer Support for Voice and Video Calls. Checked 2026-09-25.
[^19]: Cisco recommends early offer on the Unified CM trunk used for Webex Edge Audio but states Edge Audio does not require it and also supports delayed offer. Source: [Cisco Webex Edge Audio Customer Configuration Guide](https://help.webex.com/en-us/article/xmsy7d/Cisco-Webex-Edge-Audio-Customer-Configuration-Guide), (Optional) Set Early Offer Support for Voice and Video Calls. Checked 2026-09-25.
[^20]: Because Best Effort (no MTP inserted) never adds an MTP, a Unified CM trunk set that way cannot guarantee an Early Offer when the call arrives from a Delayed Offer source, so a peer that rejects Delayed Offer needs either the Mandatory (insert MTP if needed) option or DO-EO conversion downstream (inferred). Source: [Cisco Unified Communications Manager Trunks (Session Management Edition deployment guide 8.6(1))](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/session_mgmt/deploy/8_6_1/trunks.html), SIP Delayed Offer and Early Offer > SIP Early Offer support; combined with cisco-help-xmsy7d-edge-audio-config section '(Optional) Set Early Offer Support for Voice and Video Calls'. Checked 2026-09-25.
[^21]: A Unified CM call that reaches Teams Direct Routing as a Delayed Offer will not be supported, so the SBC in front of Teams must either receive an Early Offer from Unified CM or convert Delayed Offer to Early Offer itself (for example with CUBE early-offer forced) (inferred). Source: [Teams Phone System Direct Routing: SIP protocol](https://learn.microsoft.com/en-us/microsoftteams/direct-routing-protocols-sip), Inbound call: SIP dialog description > Non-media bypass flow (Delayed Offer note); combined with cisco-cube-xe-do-to-eo feature overview. Checked 2026-09-25.
[^22]: Cisco's Webex Calling Local Gateway IOS XE configuration includes early-offer forced under the global voice service voip configuration in the registration-based trunk procedure. Source: [Configure Local Gateway on Cisco IOS XE for Webex Calling](https://help.webex.com/en-us/article/jr1i3r/Configure-Local-Gateway-on-Cisco-IOS-XE-for-Webex-Calling), Configure Webex Calling registration-based trunk > voice service voip block. Checked 2026-09-25.
[^23]: The Local Gateway guide explains early-offer forced as forcing the gateway to send SDP in the initial INVITE rather than waiting for the neighbouring peer. Source: [Configure Local Gateway on Cisco IOS XE for Webex Calling](https://help.webex.com/en-us/article/jr1i3r/Configure-Local-Gateway-on-Cisco-IOS-XE-for-Webex-Calling), Configure Webex Calling registration-based trunk > command explanations for voice service voip. Checked 2026-09-25.
[^24]: In SIP an initial INVITE may carry an SDP offer in its body but is not required to; an INVITE without an offer is permitted by RFC 3261. Source: [RFC 3261 — SIP: Session Initiation Protocol](https://www.rfc-editor.org/rfc/rfc3261.html), Section 13.2.1 Creating the Initial INVITE. Checked 2026-09-25.
[^25]: If the initial INVITE contains no offer, the offer is carried in the first reliable non-failure response from the UAS (the 2xx in base RFC 3261) and the answer must then be carried in the ACK. Source: [RFC 3261 — SIP: Session Initiation Protocol](https://www.rfc-editor.org/rfc/rfc3261.html), Section 13.2.1 Creating the Initial INVITE. Checked 2026-09-25.
[^26]: A UAS that receives an INVITE without an offer must generate an offer in a reliable response. Source: [RFC 3261 — SIP: Session Initiation Protocol](https://www.rfc-editor.org/rfc/rfc3261.html), Section 13.3.1 Processing of the INVITE. Checked 2026-09-25.
[^27]: When a UAC that sent an INVITE without an offer receives the offer in a reliable provisional response, it answers in the PRACK. Source: [RFC 3262: Reliability of Provisional Responses in the Session Initiation Protocol (SIP)](https://www.rfc-editor.org/rfc/rfc3262.html), Section 5 The Offer/Answer Model and PRACK. Checked 2026-09-25.
[^28]: When the INVITE contained no offer and a reliable provisional response is the first reliable message sent back to the UAC, the offer must appear in that reliable provisional response. Source: [RFC 3262: Reliability of Provisional Responses in the Session Initiation Protocol (SIP)](https://www.rfc-editor.org/rfc/rfc3262.html), Section 5 The Offer/Answer Model and PRACK. Checked 2026-09-25.
[^29]: RFC 6337 acknowledges that its recommended behaviour for offers in response to offerless INVITEs is not always achievable, for example in some interworking scenarios. Source: [RFC 6337: Session Initiation Protocol (SIP) Usage of the Offer/Answer Model](https://www.rfc-editor.org/rfc/rfc6337.html), Section 5.2.5 Subsequent Offers and Answers. Checked 2026-09-25.
[^30]: For an INVITE without SDP, RFC 6337 describes the UAS offer as going in the first reliable 1xx response (answered in PRACK) or, if none, in the final 2xx response, and says the UAS ought not include SDP in other responses to that INVITE. Source: [RFC 6337: Session Initiation Protocol (SIP) Usage of the Offer/Answer Model](https://www.rfc-editor.org/rfc/rfc6337.html), Section 3.1.2 INVITE Request without SDP. Checked 2026-09-25.
[^31]: RFC 6337 recommends that an offer sent in response to an offerless (re-)INVITE include all codecs the UA is currently willing and able to use, not only those negotiated in earlier exchanges. Source: [RFC 6337: Session Initiation Protocol (SIP) Usage of the Offer/Answer Model](https://www.rfc-editor.org/rfc/rfc6337.html), Section 5.2.5 Subsequent Offers and Answers. Checked 2026-09-25.
[^32]: In Unified CM 8.6(1), SIP trunks used Delayed Offer (INVITE without SDP) by default. Source: [Cisco Unified Communications Manager Trunks (Session Management Edition deployment guide 8.6(1))](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/session_mgmt/deploy/8_6_1/trunks.html), SIP Delayed Offer and Early Offer. Checked 2026-09-25.
[^33]: The Unified CM option 'Early Offer support for voice and video calls (insert MTP if needed)' inserts an MTP only when the calling device cannot supply the media characteristics needed to build the Early Offer. Source: [Cisco Unified Communications Manager Trunks (Session Management Edition deployment guide 8.6(1))](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/session_mgmt/deploy/8_6_1/trunks.html), SIP Delayed Offer and Early Offer > Early Offer support for voice and video calls. Checked 2026-09-25.
[^34]: On a Unified CM 8.6(1) SIP trunk, Media Termination Point Required statically assigns an MTP to every outbound call and supports only G.711 or G.729, limiting the trunk to voice calls. Source: [Cisco Unified Communications Manager Trunks (Session Management Edition deployment guide 8.6(1))](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/session_mgmt/deploy/8_6_1/trunks.html), SIP Delayed Offer and Early Offer > Media Termination Point Required. Checked 2026-09-25.
[^35]: Under insert-MTP-if-needed Early Offer, Unified CM 8.6(1) inserts an MTP for calls from older SCCP phones and for calls arriving on SIP Delayed Offer or H.323 Slow Start trunks that are extended to an Early Offer trunk. Source: [Cisco Unified Communications Manager Trunks (Session Management Edition deployment guide 8.6(1))](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/session_mgmt/deploy/8_6_1/trunks.html), SIP Delayed Offer and Early Offer > SIP Early Offer support. Checked 2026-09-25.
[^36]: Under insert-MTP-if-needed Early Offer, Unified CM 8.6(1) needs no MTP for calls arriving on SIP Early Offer trunks, H.323 Fast Start trunks or MGCP trunks, or originating from SIP phones or newer SCCP phones. Source: [Cisco Unified Communications Manager Trunks (Session Management Edition deployment guide 8.6(1))](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/session_mgmt/deploy/8_6_1/trunks.html), SIP Delayed Offer and Early Offer > SIP Early Offer support. Checked 2026-09-25.
[^37]: In Direct Routing non-bypass mode a Teams media answer is converted to a 183 Session Progress whose SDP is replaced by the Media Processor's SDP, and only one 183 per session is sent. Source: [Teams Phone System Direct Routing: SIP protocol](https://learn.microsoft.com/en-us/microsoftteams/direct-routing-protocols-sip), Inbound call: SIP dialog description (comparison table) and Multiple endpoints ringing with provisional answer, step 4. Checked 2026-09-25.
[^38]: On a Direct Routing inbound call, the Teams 'Call accepted' event is converted by the SIP proxy into a 200 response with SDP, and the SBC is expected to send and receive media to the candidates in that SDP. Source: [Teams Phone System Direct Routing: SIP protocol](https://learn.microsoft.com/en-us/microsoftteams/direct-routing-protocols-sip), Inbound call: SIP dialog description > Non-media bypass flow. Checked 2026-09-25.
[^39]: When a media-bypass Direct Routing call is transferred to a client that cannot use media bypass, Direct Routing starts an ICE restart by changing ice-pwd and ice-ufrag and offering new media candidates in a re-INVITE. Source: [Teams Phone System Direct Routing: SIP protocol](https://learn.microsoft.com/en-us/microsoftteams/direct-routing-protocols-sip), ICE Restart: Media bypass call transferred to an endpoint that doesn't support media bypass. Checked 2026-09-25.
[^40]: Microsoft Teams Direct Routing does not support Delayed Offer INVITEs (INVITE without SDP). Source: [Teams Phone System Direct Routing: SIP protocol](https://learn.microsoft.com/en-us/microsoftteams/direct-routing-protocols-sip), Inbound call: SIP dialog description > Non-media bypass flow > note under 'Call accepted'. Checked 2026-09-25.
[^41]: Direct Routing may send SIP messages larger than 1,500 bytes, mainly because of SDP, and Microsoft recommends the SBC strip SDP values such as ICE candidates or unused codecs before forwarding to UDP trunks. Source: [Teams Phone System Direct Routing: SIP protocol](https://learn.microsoft.com/en-us/microsoftteams/direct-routing-protocols-sip), Size of SDP considerations. Checked 2026-09-25.
[^42]: CUBE requires 100rel with rel1xx require 100rel under voice service voip > sip globally, or voice-class sip rel1xx require 100rel on a dial peer, and the dial-peer setting takes precedence. Source: [SIP Reliable Provisional Response on CUBE and CUCM Configuration Example](https://www.cisco.com/c/en/us/support/docs/voice/session-initiation-protocol-sip/116086-configure-cube-cucm-sip-00.html), Configure > CUBE. Checked 2026-09-25.
[^43]: As documented in 2013, CUCM supported reliable provisional responses only as a UAC sending PRACK and could not itself send 180/183 with Require: 100rel as a UAS. Source: [SIP Reliable Provisional Response on CUBE and CUCM Configuration Example](https://www.cisco.com/c/en/us/support/docs/voice/session-initiation-protocol-sip/116086-configure-cube-cucm-sip-00.html), Configure > CUCM (note on UAS support). Checked 2026-09-25.
[^44]: When CUCM places a call with a Delayed Offer, the caller may not hear PSTN-generated ringback or announcements because no media path is established before the call is answered. Source: [SIP Reliable Provisional Response on CUBE and CUCM Configuration Example](https://www.cisco.com/c/en/us/support/docs/voice/session-initiation-protocol-sip/116086-configure-cube-cucm-sip-00.html), Introduction / background. Checked 2026-09-25.
[^45]: Cisco documents SIP reliable provisional responses (PRACK/100rel) as a way to establish early media on delayed-offer calls without using extra media resources such as MTPs. Source: [SIP Reliable Provisional Response on CUBE and CUCM Configuration Example](https://www.cisco.com/c/en/us/support/docs/voice/session-initiation-protocol-sip/116086-configure-cube-cucm-sip-00.html), Introduction / background. Checked 2026-09-25.
[^46]: In Unified CM the SIP Profile (Device > Device Settings > SIP Profile) has a SIP Rel1XX Options setting that can be set to send PRACK for 1xx messages. Source: [SIP Reliable Provisional Response on CUBE and CUCM Configuration Example](https://www.cisco.com/c/en/us/support/docs/voice/session-initiation-protocol-sip/116086-configure-cube-cucm-sip-00.html), Configure > CUCM. Checked 2026-09-25.
[^47]: Cisco's CUCM-to-CUBE configuration example recommends sending Early Offer from CUCM to avoid early media cut-through issues. Source: [Configure CUCM to CUBE Integration - Configuration Example](https://www.cisco.com/c/en/us/support/docs/unified-communications/unified-communications-manager-callmanager/117300-configure-cube-00.html), Configure > CUCM SIP Profile configuration. Checked 2026-09-25.
[^48]: Cisco's CUCM-to-CUBE configuration example (2014) states that almost all service providers require Early Offer SIP calls. Source: [Configure CUCM to CUBE Integration - Configuration Example](https://www.cisco.com/c/en/us/support/docs/unified-communications/unified-communications-manager-callmanager/117300-configure-cube-00.html), Configure > CUBE voice service voip configuration (early-offer forced discussion). Checked 2026-09-25.