Source record · tier 2 current vendor documentation

Elastic SIP Trunking

Publisher: Twilio
URL: https://www.twilio.com/docs/sip-trunking
Published: 2026-07-21
Updated: unknown
Accessed: 2026-09-24
HTTP status: 200
License: Twilio documentation terms; no-redistribution; short excerpts and locators only

Source notes citing this source

The addresses placed in a Twilio IP access control list for termination should be the IP addresses your infrastructure uses for outbound SIP traffic. in context
When the trunk's Call Transfer option is enabled, Twilio consumes a SIP REFER from the customer infrastructure and creates a new INVITE. in context
The caller ID number on a termination call must be a Twilio number on the account or a caller ID verified in the Twilio Console. in context
If no caller ID number is in the INVITE From field, Twilio uses the Remote-Party-ID or P-Asserted-Identity header instead. in context
Twilio highly recommends configuring credential authentication for termination because IP access control lists alone do not protect against certain types of attacks. in context
A trunk can have a Disaster Recovery URL pointing to a TwiML application that handles calls Twilio cannot deliver to the origination URIs. in context
Phone numbers sent to Twilio over SIP must be E.164-formatted, such as +12128675309, or Twilio rejects the call with SIP 400 Bad Request. in context
Twilio offers localized termination URIs that pin traffic to an edge, such as {example}.pstn.ashburn.twilio.com for North America Virginia, {example}.pstn.umatilla.twilio.com for Oregon, {example}.pstn.dublin.twilio.com for Ireland and {example}.pstn.frankfurt.twilio.com for Frankfurt. in context
Twilio instructs customers to configure their infrastructure not to register for an Elastic SIP Trunking trunk. in context
At least one Twilio phone number must be associated with the trunk before it can receive incoming PSTN calls. in context
Twilio sends originating SIP requests over UDP by default, and a transport parameter such as ;transport=tcp or ;transport=tls on the origination URI selects another transport. in context
A Twilio trunk accepts up to 10 origination URIs, each with a priority from 0 to 65535 where the lowest number is most important, and a weight that shares load among URIs of equal priority. in context
A Twilio origination SIP URI names where Twilio sends inbound calls, in forms such as sip:172.56.42.132 or sip:mysbc.com. in context
When Secure Trunking is enabled on a Twilio trunk, any non-encrypted call is rejected. in context
Twilio negotiates SRTP keys using SDP Security Descriptions (SDES) as defined in RFC 4568. in context
Twilio supports only the AES_CM_128_HMAC_SHA1_80 SRTP suite on origination calls, and both AES_CM_128_HMAC_SHA1_80 and AES_CM_128_HMAC_SHA1_32 on termination calls. in context
When the trunk's Symmetric RTP option is enabled, Twilio sends RTP to the address the remote stream actually arrives from rather than the one negotiated in SDP. in context
A Twilio trunk's termination must have at least an IP access control list or credential authentication configured, and if both are configured both are enforced. in context
A Twilio Elastic SIP Trunking termination SIP URI takes the form {example}.pstn.twilio.com, and Twilio recommends dashes rather than dots in the chosen name. in context
With Secure Trunking, TLS uses port 5061 by default, another port may be given in the origination URI, and the origination URI must carry the transport=tls parameter. in context
Twilio Secure Trunking supports TLS 1.2 and above for SIP signaling. in context
Devices that communicate directly with Twilio over TLS must be configured to trust Twilio's TLS certificate, for which Twilio publishes root CA certificates. in context
Twilio Elastic SIP Trunking accepts up to ten origination SIP URIs whose host is an IP address or FQDN, each with a priority (0-65535, lowest used first) and a weight (1-65535) for sharing load within a priority. in context
Twilio Elastic SIP Trunking has a per-trunk Symmetric RTP setting. When enabled, Twilio latches onto the incoming RTP stream and sends RTP back to where it came from, not to the SDP-negotiated address. Twilio offers this for customers behind a non-SIP-aware NAT whose SDP carries a private IP. in context
Twilio says that with Symmetric RTP disabled it sends RTP to the destination negotiated in the SDP, and that this setting is considered more secure and is therefore recommended. in context
Source-IP trust (the CUBE trusted list, Oracle ACLs, Twilio IP ACLs) and digest or TLS credentials are complementary controls, and vendors differ on whether IP trust alone is enough: Cisco documents IP trust as its primary toll-fraud control, while Twilio warns that IP ACLs alone are insufficient. inferred in context
Twilio Elastic SIP Trunking requires at least an IP access control list or a credential list for termination, and enforces both when both are set. in context
Twilio strongly recommends configuring user credentials on SIP trunks because IP ACLs alone do not protect against certain types of attack. in context
Twilio Elastic SIP Trunking termination requires at least an IP access control list or a credential list, and enforces both when both are configured. in context
Twilio sizes peak SIP trunk bandwidth as maximum simultaneous calls times 100 kbps, a figure covering G.711 plus overhead. in context
On a Twilio Elastic SIP Trunking termination call, the Caller ID number must be a Twilio DID on the account or a number verified in the Twilio Console. in context
Twilio strongly recommends configuring credentials on a termination trunk and says IP ACLs alone do not protect against certain types of attack. in context
Twilio Elastic SIP Trunking accepts up to ten origination SIP URIs, ordered by priority (lower number is preferred) with weight splitting load among equal priorities, and fails over to the next URI after 4 seconds without a SIP response. in context
When a call cannot be delivered to any origination SIP URI, Twilio invokes the trunk's Disaster Recovery URL, a TwiML application that can play a message, take voicemail, redirect to another number or replicate PBX functions such as an IVR. in context
Calls redirected to a Twilio Elastic SIP Trunking Disaster Recovery URL are billed at normal Twilio Voice rates. in context
If an origination SIP server gives no SIP response, Twilio fails over to the next URI after 4 seconds. in context
A Twilio Elastic SIP Trunk can have up to 10 origination SIP URIs; Twilio uses the lowest priority value first, falls back to higher values if the session fails, and uses weight to share load among URIs of equal priority. in context

Cite this source record

APA

WarmTransfer. (2026, July 21). Elastic SIP Trunking. WarmTransfer. https://warmtransfer.net/knowledge/sources/twilio-docs-elastic-sip-trunking

BibTeX

@misc{warmtransfer-twilio-docs-elastic-sip-trunking,
  title  = {Elastic SIP Trunking},
  author = {{WarmTransfer}},
  year   = {2026},
  url    = {https://warmtransfer.net/knowledge/sources/twilio-docs-elastic-sip-trunking},
  note   = {Twilio, accessed 2026-09-24}
}