Quick question before I dive into setup — is it po...
# ❓・ai-evy-supports
Suf
06/17/2026, 9:30 PM@AI Evy Quick question before I dive into setup — is it possible to use an existing UK number (that we own) as the outbound caller ID for API-triggered calls via a 3CX SIP trunk, without having to purchase a number through Retell or Twilio?
Context: we're currently using another AI calling platform and ran into a limitation where SIP trunk numbers can't be used as the 'from' caller ID for self-serve users. Wanted to check if this is something Retell handles differently before committing to a switch.
a
AI Evy
06/17/2026, 9:30 PMThanks for tagging me @Suf. I'll look through relevant sources and get back to you shortly.
AI Evy
06/17/2026, 9:30 PM### Answer
Yes, you can use your existing UK number as the outbound caller ID through Retell's Elastic SIP Trunking integration with your 3CX SIP trunk. [(1) ]()
Retell supports custom telephony via SIP trunking, which allows you to import your existing numbers and use them for both inbound and outbound calls. When you import a number through the SIP trunk method, you'll need to provide your termination URI (which would be your 3CX SIP trunk endpoint) and authentication credentials. [(2) ]()
For outbound calls specifically, you can configure caller ID masking through SIP header manipulation. While the documentation shows Twilio-specific examples of setting up verified caller IDs and using SIP header rules to display custom caller IDs, [(3) ]() the same principle applies to other SIP trunk providers like 3CX.
The key steps would be:
1. Set up your 3CX SIP trunk with proper termination and origination URIs [(1) ]()
2. Import your UK number into Retell using the Import Phone Number API, providing your 3CX termination URI and authentication details [(2) ]()
3. Configure your SIP trunk to include the appropriate caller ID in the SIP headers for outbound calls
Once imported, you can make outbound calls using this number through the Create Phone Call API, and it will display your UK number as the caller ID. [(2) ]() [(1) ]()
AI Evy
06/17/2026, 9:30 PMFeel free to tag @AI Evy with additional questions. I also appreciate feedback, it helps me improve.
AI Evy
06/17/2026, 9:30 PM
To learn more, see the sources I considered:
[[1] Custom Telephony Overview](https://docs.retellai.com/deploy/custom-telephony)
[[2] Import Phone Number](https://docs.retellai.com/api-references/import-phone-number)
[[3] Twilio](https://docs.retellai.com/deploy/twilio)
s
Suf
06/17/2026, 9:31 PM@AI Evy Is there a human agent I can verify this info with?
a
AI Evy
06/17/2026, 9:32 PMThanks for the follow-up, @Suf. I'll get back to you soon.
AI Evy
06/17/2026, 9:32 PM
To learn more, see the sources I considered:
[[1] Custom Telephony Overview](https://docs.retellai.com/deploy/custom-telephony)
AI Evy
06/17/2026, 9:32 PMFeel free to tag @AI Evy with additional questions. I also appreciate feedback, it helps me improve.