DTMF methods and interoperability failures

See also

Sources

1. 1
   In Asterisk, dtmf_mode auto sends RFC 4733 if the other side supports it and in-band otherwise; auto_info sends RFC 4733 if supported and SIP INFO otherwise.

   res_pjsip - Asterisk Documentation · endpoint > dtmf_mode > auto, auto_info · Checked 2026-09-24
2. 2
   Asterisk res_pjsip endpoint dtmf_mode defaults to rfc4733 and accepts rfc4733, inband, none, info, auto and auto_info.

   res_pjsip - Asterisk Documentation · endpoint > dtmf_mode · Checked 2026-09-24
3. 3
   Cisco's CUBE technote recommends RFC 2833 toward SIP service providers and both RFC 2833 and KPML toward Unified CM or CME.

   Configure DTMF Relay on CUBE · Preferred DTMF methods by scenario · Checked 2026-09-24
4. 4
   On CUBE, interworking RFC 2833 with G.711 in-band audio DTMF requires DSP transcoding resources and G.711 end to end; Cisco states the other DTMF interworking cases are handled without a transcoder in flow-through calls.

   Configure DTMF Relay on CUBE · DTMF interworking section; G711 to RFC2833 requirements · Checked 2026-09-24
5. 5
   Cisco's proprietary cisco-rtp DTMF relay (payload type 121) has been discontinued on CUBE.

   Cisco Unified Border Element Configuration Guide - Cisco IOS XE 17.6 Onwards - DTMF Relay · DTMF Relay chapter, cisco-rtp description · Checked 2026-09-24
6. 6
   Cisco's fix for duplicate digits is to configure dtmf-relay rtp-nte digit-drop on the inbound dial-peer and the out-of-band method on the outbound dial-peer, so CUBE suppresses NTE packets and relays only the out-of-band digits.

   Configure DTMF Relay on CUBE · Digit-drop section · Checked 2026-09-24
7. 7
   When CUBE interworks rtp-nte to an out-of-band method without digit-drop, the same digit is sent both in RTP and out-of-band and the far end sees duplicate digits.

   Configure DTMF Relay on CUBE · Digit-drop section · Checked 2026-09-24
8. 8
   CUBE uses payload types 96 and 97 for fax by default, and Cisco advises the asymmetric payload dtmf command when a third-party device's DTMF payload type conflicts.

   Cisco Unified Border Element Configuration Guide - Cisco IOS XE 17.6 Onwards - DTMF Relay · DTMF Relay chapter, payload type note · Checked 2026-09-24
9. 9
   H.323 (and with it the H.245 alphanumeric and signal DTMF methods) is not supported on CUBE from IOS XE Bengaluru 17.6.1a onwards.

   Cisco Unified Border Element Configuration Guide - Cisco IOS XE 17.6 Onwards - DTMF Relay · DTMF Relay chapter, restriction note on H.323 · Checked 2026-09-24
10. 10
    On CUBE (IOS XE 17.6 onwards), SIP DTMF relay methods are rtp-nte (RFC 2833), sip-notify (Cisco proprietary), sip-kpml (RFC 4730) and sip-info, plus G.711 in-band audio, which needs no configuration.

    Cisco Unified Border Element Configuration Guide - Cisco IOS XE 17.6 Onwards - DTMF Relay · DTMF Relay chapter, supported methods list · Checked 2026-09-24
11. 11
    If an endpoint supports none of the DTMF relay methods configured on CUBE, an MTP or transcoder is required.

    Cisco Unified Border Element Configuration Guide - Cisco IOS XE 17.6 Onwards - DTMF Relay · DTMF Relay chapter, fallback/restrictions · Checked 2026-09-24
12. 12
    When more than one out-of-band DTMF method is configured on a CUBE dial-peer, preference runs from highest to lowest in the order the methods were configured.

    Cisco Unified Border Element Configuration Guide - Cisco IOS XE 17.6 Onwards - DTMF Relay · DTMF Relay chapter, dtmf-relay command notes · Checked 2026-09-24
13. 13
    On CUBE, DTMF relay is disabled by default on SIP and H.323 dial-peers except SIP INFO, so a relay method has to be configured.

    Configure DTMF Relay on CUBE · Section on CUBE supported DTMF methods / configuration · Checked 2026-09-24
14. 14
    CUBE prefers sip-notify or sip-kpml over rtp-nte, and rtp-nte over in-band, when selecting a DTMF method.

    Cisco Unified Border Element Configuration Guide - Cisco IOS XE 17.6 Onwards - DTMF Relay · DTMF Relay chapter, DTMF relay selection/priority description · Checked 2026-09-24
15. 15
    On CUBE, sip-info DTMF handling is always enabled for SIP dial-peers and is invoked when a SIP INFO with DTMF relay content is received.

    Cisco Unified Border Element Configuration Guide - Cisco IOS XE 17.6 Onwards - DTMF Relay · DTMF Relay chapter, sip-info description · Checked 2026-09-24
16. 16
    A Cisco IOS XE gateway can receive SIP INFO with Content-Type application/dtmf-relay carrying Signal and Duration fields and play the DTMF tone on the telephony call leg.

    CUBE Configuration Guide - Cisco IOS XE 17.6 Onwards - SIP INFO Method for DTMF Tone Generation · SIP INFO Method for DTMF Tone Generation, feature information · Checked 2026-09-24
17. 17
    The CUBE dtmf-relay command accepts up to six methods on a VoIP dial-peer.

    Cisco Unified Border Element Configuration Guide - Cisco IOS XE 17.6 Onwards - DTMF Relay · DTMF Relay chapter, dtmf-relay method1 [...[method6]] syntax · Checked 2026-09-24
18. 18
    Unified CM inserts an MTP when a SIP endpoint supports only RFC 2833 and the other side needs out-of-band DTMF; the MTP exchanges RFC 2833 packets with the SIP endpoint and out-of-band DTMF with Unified CM.

    CallManager/ DTMF/ MTP Allocation · DTMF/MTP allocation table and explanation · Checked 2026-09-24
19. 19
    Cisco's CUBE DTMF technote (updated May 2023) states that Unified CM does not support SIP INFO for DTMF.

    Configure DTMF Relay on CUBE · SIP INFO section · Checked 2026-09-24
20. 20
    The default RTP clock rate for the telephone-event payload is 8000 Hz, which can be redefined when the dynamic payload type is assigned.

    RFC 4733: RTP Payload for DTMF Digits, Telephony Tones, and Telephony Signals · Section 2.1 · Checked 2026-09-24
21. 21
    RFC 4733 maps DTMF digits 0-9 to event codes 0-9, star to 10, pound to 11, and A-D to 12-15.

    RFC 4733: RTP Payload for DTMF Digits, Telephony Tones, and Telephony Signals · Section 3.2 · Checked 2026-09-24
22. 22
    The telephone-event payload format has no static RTP payload type number; the payload type is assigned dynamically and signalled out-of-band (for example in SDP).

    RFC 4733: RTP Payload for DTMF Digits, Telephony Tones, and Telephony Signals · Section 2.1 · Checked 2026-09-24
23. 23
    The final (E-bit) packet for each telephone event and each segment SHOULD be sent a total of three times at the source's update interval.

    RFC 4733: RTP Payload for DTMF Digits, Telephony Tones, and Telephony Signals · Section 2.5.1.4 Retransmission of Final Packet · Checked 2026-09-24
24. 24
    If no 'events' fmtp parameter is received, an RFC 4733 sender SHOULD assume the peer supports DTMF events 0-15 and no other events.

    RFC 4733: RTP Payload for DTMF Digits, Telephony Tones, and Telephony Signals · Section 2.5.1.1 Negotiation of Payloads · Checked 2026-09-24
25. 25
    The first RTP packet for a telephone event MUST have the marker (M) bit set.

    RFC 4733: RTP Payload for DTMF Digits, Telephony Tones, and Telephony Signals · Section 2.5.1.2 Transmission of Event Packets · Checked 2026-09-24
26. 26
    RFC 4733 recommends that a gateway render only the telephone-event payload once it is received, because the concurrent audio may contain spurious tones introduced by compression.

    RFC 4733: RTP Payload for DTMF Digits, Telephony Tones, and Telephony Signals · Section 2.5.2.2 Playout of Tone Events · Checked 2026-09-24
27. 27
    Genesys lists out-of-band DTMF (RFC 2833) as a required capability for BYOC SIP providers, and says customer endpoints should handle in-band if RFC 2833 is not supported by the PBX or far end.

    SIP provider configuration information and questionnaire for BYOC Cloud/BYOC Premises trunks · Functionality requirements; RFCs supported (RFC 2833 row) · Checked 2026-09-24
28. 28
    Genesys Cloud external (BYOC) trunks offer three DTMF methods: RTP Events (default, RFC 2833/4733), In-band Audio, and None.

    External trunk settings · DTMF settings > DTMF Method · Checked 2026-09-24
29. 29
    The Genesys Cloud external trunk DTMF Payload setting accepts 96-127, defaults to 101, and applies only when the DTMF Method is RTP Events.

    External trunk settings · DTMF settings > DTMF Payload · Checked 2026-09-24
30. 30
    In-band DTMF is recognised reliably only on G.711 (ulaw or alaw); low-bit-rate codecs distort the tones so they may not be recognised.

    Configure DTMF Relay on CUBE · Common issues: codec distortion / G711 requirements · Checked 2026-09-24
31. 31
    RFC 4730 (November 2006) defines the 'kpml' SIP event package, in which a subscriber sends digit patterns in a SUBSCRIBE and matched key presses are reported in NOTIFY messages.

    RFC 4730: A Session Initiation Protocol (SIP) Event Package for Key Press Stimulus (KPML) · Sections 2 and 4 · Checked 2026-09-24
32. 32
    KPML digit-pattern matching uses longest-match rather than first-match semantics.

    RFC 4730: A Session Initiation Protocol (SIP) Event Package for Key Press Stimulus (KPML) · Section 3.3 · Checked 2026-09-24
33. 33
    RFC 6086 keeps a 'legacy' mode of INFO usage compatible with RFC 2976 for backward compatibility.

    RFC 6086: Session Initiation Protocol (SIP) INFO Method and Package Framework · Section 3 · Checked 2026-09-24
34. 34
    RFC 6086 attributes industry interoperability problems to legacy INFO usages that require static configuration.

    RFC 6086: Session Initiation Protocol (SIP) INFO Method and Package Framework · Section 2 · Checked 2026-09-24
35. 35
    If an event lasts beyond the maximum duration field value 0xFFFF, the sender MUST send a packet reporting that maximum without the E bit and continue the event in a new segment.

    RFC 4733: RTP Payload for DTMF Digits, Telephony Tones, and Telephony Signals · Section 2.5.1.3 Long-Duration Events · Checked 2026-09-24
36. 36
    Cisco documentation classifies RFC 2833 NTE as an in-band DTMF method because the events travel in the RTP stream.disputed

    Cisco Unified Border Element Configuration Guide - Cisco IOS XE 17.6 Onwards - DTMF Relay · DTMF Relay chapter, In-Band methods list (rtp-nte) · Checked 2026-09-24
37. 37
    Asterisk documentation describes rfc4733 DTMF as sent out of band of the main audio stream, and Genesys describes RTP Events as out-of-band.disputed

    res_pjsip - Asterisk Documentation · endpoint > dtmf_mode > rfc4733 · Checked 2026-09-24
38. 38
    PJSIP documentation warns that if the remote side does not support SIP INFO and never replies, the sender treats it as a transaction timeout and disconnects the call.

    DTMF — PJSIP Project documentation · Sending DTMF · Checked 2026-09-24
39. 39
    PJSIP generates in-band DTMF tones but does not implement in-band DTMF detection.

    DTMF — PJSIP Project documentation · Overview · Checked 2026-09-24
40. 40
    In PJSIP, sending RFC 4733 DTMF when the negotiated SDP lacks the telephone-event rtpmap returns PJMEDIA_RTP_EREMNORFC2833; any fallback is up to the application.

    DTMF — PJSIP Project documentation · Overview · Checked 2026-09-24
41. 41
    PJSIP documentation calls RFC 4733 telephone-event the recommended, standards-based DTMF method.

    DTMF — PJSIP Project documentation · Overview · Checked 2026-09-24
42. 42
    Payload type 101 for telephone-event is a common vendor default rather than a value fixed by the standard, so a far end may legitimately negotiate a different dynamic value.inferred

    RFC 4733: RTP Payload for DTMF Digits, Telephony Tones, and Telephony Signals · Section 2.1 (dynamic payload type), read with Genesys external trunk settings DTMF Payload default 101 · Checked 2026-09-24
43. 43
    Under RFC 6086 a UA supporting Info Packages MUST list the packages it will accept in the Recv-Info header field.

    RFC 6086: Session Initiation Protocol (SIP) INFO Method and Package Framework · Section 5.2.2 · Checked 2026-09-24
44. 44
    RFC 2833 (May 2000, Proposed Standard) is obsoleted by two documents: RFC 4733 and RFC 4734.

    Information on RFC 2833: RTP Payload for DTMF Digits, Telephony Tones and Telephony Signals · RFC Editor info page, Obsoleted by field · Checked 2026-09-24
45. 45
    RFC 4733 places the duty to avoid double detection mainly on the receiving gateway; the checked sections contain no requirement that a sender mute in-band tones while sending events, so duplicate-digit protection cannot be assumed from the sender.inferred

    RFC 4733: RTP Payload for DTMF Digits, Telephony Tones, and Telephony Signals · Sections 2.5.1 and 2.5.2.2 (absence of a sender muting requirement in the sections checked) · Checked 2026-09-24
46. 46
    RFC 4733, published December 2006 on the Standards Track, obsoletes RFC 2833.

    RFC 4733: RTP Payload for DTMF Digits, Telephony Tones, and Telephony Signals · RFC 4733 header block (Obsoletes: 2833; Category: Standards Track; December 2006) · Checked 2026-09-24
47. 47
    RFC 6086 (January 2011) defines the SIP INFO method and Info Package framework and obsoletes RFC 2976.

    RFC 6086: Session Initiation Protocol (SIP) INFO Method and Package Framework · RFC 6086 header block · Checked 2026-09-24
48. 48
    RFC 6086 states that DTMF transport over SIP INFO has been done with proprietary, unstandardised mechanisms.

    RFC 6086: Session Initiation Protocol (SIP) INFO Method and Package Framework · Section 2 (Motivation) · Checked 2026-09-24
49. 49
    For SIP INFO tone generation the Cisco gateway clamps Duration to 100-5000 ms and uses 250 ms if no duration is given.

    CUBE Configuration Guide - Cisco IOS XE 17.6 Onwards - SIP INFO Method for DTMF Tone Generation · SIP INFO Method for DTMF Tone Generation, duration parameters · Checked 2026-09-24
50. 50
    For media bypass early media, Microsoft says the SBC should support sending DTMF during the early-media phase to enable IVR and voicemail scenarios.

    Direct Routing support for media bypass - Microsoft Teams · Scenarios > Inbound call from SBC > Early media · Checked 2026-09-24
51. 51
    The Teams Phone Direct Routing media stack does not support in-band DTMF.

    Teams Phone Direct Routing: Definitions and RFC standards · Section 'DTMF' · Checked 2026-09-24
52. 52
    The Direct Routing 'RFC standards' list that SBCs must comply with does not name RFC 2833 or RFC 4733, so the page does not itself specify which out-of-band DTMF method or payload type Teams expects.inferred

    Teams Phone Direct Routing: Definitions and RFC standards · Section 'RFC standards' and subsections (absence) · Checked 2026-09-24
53. 53
    When a PSTN leg delivers DTMF only as in-band audio, the Direct Routing SBC has to convert it to a non-in-band method before it reaches Teams, or Teams IVRs and voicemail will not see the digits.inferred

    Teams Phone Direct Routing: Definitions and RFC standards · Section 'DTMF' combined with RFC 4733 Section 2.5.2.2 · Checked 2026-09-24
54. 54
    The RFC 4733 payload format for named telephone events is designated 'telephone-event' with media type audio/telephone-event.

    RFC 4733: RTP Payload for DTMF Digits, Telephony Tones, and Telephony Signals · Section 2.1 · Checked 2026-09-24
55. 55
    RFC 4733 allows a source to choose its own spacing for event update packets and recommends 50 ms.

    RFC 4733: RTP Payload for DTMF Digits, Telephony Tones, and Telephony Signals · Section 2.5.1.2 Transmission of Event Packets · Checked 2026-09-24
56. 56
    The RFC 4733 volume field expresses power level for tone events from 0 to -63 dBm0.

    RFC 4733: RTP Payload for DTMF Digits, Telephony Tones, and Telephony Signals · Section 2.3.4 Volume Field · Checked 2026-09-24
57. 57
    Cisco's Webex Calling Local Gateway configuration uses dtmf-relay rtp-nte on the PSTN-facing dial-peer and on the Unified CM-facing dial-peers.

    Configure Local Gateway on Cisco IOS XE for Webex Calling · Registration-based LGW: dial-peer 200 (IP PSTN and TDM); Unified CM integration: dial-peers 300 and 400 · Checked 2026-09-24

Documents

Cite this page

WarmTransfer. (2026, September 24). DTMF methods and interoperability failures. WarmTransfer. https://warmtransfer.net/knowledge/dtmf-interop

@misc{warmtransfer-dtmf-interop,
  title  = {DTMF methods and interoperability failures},
  author = {{WarmTransfer}},
  year   = {2026},
  url    = {https://warmtransfer.net/knowledge/dtmf-interop},
  note   = {Verified 2026-09-24}
}