Troubleshooting your Trunk

Publisher

Twilio

URL

https://www.twilio.com/docs/sip-trunking/troubleshooting

Published

2026-05-06

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

• Twilio Elastic SIP Trunking call logs in Console can be filtered by date, phone numbers, SIP addresses and call status and downloaded as CSV. in context
• For the opposite case, where the PBX user hears the PSTN party but not vice versa, Twilio gives three causes: a firewall blocking RTP from the PBX to Twilio, the internet provider filtering RTP, and routing that brings the packets' TTL down to 10 or less. in context
• Twilio's trunk troubleshooting page covers the case where the PSTN party hears the PBX user but the PBX user cannot hear the PSTN party. The causes it gives are the PBX putting its LAN IP in the SDP, and a firewall blocking RTP from Twilio. The remedies are to advertise the WAN IP in SDP and to allow RTP from Twilio's published IP ranges and ports. in context
• On Twilio origination calls, Twilio's page gives the symptom of no audio followed by the call dropping after 20 or 30 seconds. The cause it names is the customer's SIP infrastructure rewriting IP addresses in the Via headers of its response to Twilio's INVITE. in context
• Vendor use of 403 is broader than the RFC 3261 definition suggests: Twilio sends 403 for a wrong digest password and Telnyx for exceeded channel limits, conditions that a configuration change fixes, so the RFC's advice not to repeat the request is best read as applying to the unchanged request. inferred in context
• Twilio documents a '403 Phone number is blocked for verification' failure meaning Twilio has blocked the destination number, with contacting Twilio Support as the remedy if the block is unintended. in context
• For the '403 Invalid Caller ID' failure Twilio's listed fixes are to send To and From in E.164 with the plus sign, to set From to a Twilio number or verified caller ID, and to make any Remote-Party-ID in the INVITE reference a valid caller ID. in context
• Twilio documents a '403 Invalid Caller ID' failure on trunk termination calls and ties it to Free Trial accounts, which must use a Twilio-verified caller ID for both the To and From numbers. in context
• Twilio documents a '403 No International Authorization' failure that occurs when a trunk call targets a country not enabled in the account's Voice Geographic Permissions. in context
• Twilio suggests temporarily removing the Credentials List from the trunk to test whether the call succeeds with the ACL alone. in context
• Twilio documents two causes for a 403 Forbidden response to an INVITE on an Elastic SIP Trunking termination call: the source IP address is not on the trunk's ACL, or the INVITE's authentication digest does not match the trunk's Credentials List. in context
• Twilio advises checking the received parameter on the Via header of the 403 response to see the IP address from which Twilio received the SIP request. in context
• Twilio says that if the 403 answers the initial INVITE that carries no digest the problem is most likely the ACL, and if it answers the INVITE carrying an auth digest sent after a 407 the problem is most likely the credentials. in context
• For origination 408s Twilio advises confirming the trunk's Origination SIP URI and checking that the firewall and PBX allow Twilio's IP addresses and ports. in context
• Twilio states that an Elastic SIP Trunking origination call failing with 408 (or 504) Request Timeout means Twilio is getting no response from the customer's SIP infrastructure. in context

Cite this source record

APA

WarmTransfer. (2026, May 6). Troubleshooting your Trunk. WarmTransfer. https://warmtransfer.net/knowledge/sources/twilio-docs-sip-trunking-troubleshooting

BibTeX

@misc{warmtransfer-twilio-docs-sip-trunking-troubleshooting,
  title  = {Troubleshooting your Trunk},
  author = {{WarmTransfer}},
  year   = {2026},
  url    = {https://warmtransfer.net/knowledge/sources/twilio-docs-sip-trunking-troubleshooting},
  note   = {Twilio, accessed 2026-09-24}
}