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Hey everyone, I am fairly new to retell ai and swi...
# ❓・ai-evy-support
d
@AI Evy Hey everyone, I am fairly new to retell ai and switched over from synthflow. I am having an issue where when I import my own sip trunk from either twillio or telnyx an inbound ai agent to a call transfer fails if i have the setting as "users number" for displayed caller id. How can I by pass this? It works fine on a twilio number purchased from retell but not from a external sip trunk twilio or retell account. Would really appreciate if someone can reach out to help me or even a quick fix here in the chat.
a
Thanks for tagging me @Dk. I'll look through relevant sources and get back to you shortly.
If using Twilio SIP trunking, you can set up Header Manipulation policies in your trunk's Termination settings to configure a verified caller ID. [(4) ]() Unfortunately, there's nothing you can configure on the Retell side to bypass this—it's a permission/configuration issue with your telephony provider. [(3) ]()
### Answer --- This is a known limitation when using custom telephony (your own SIP trunk) versus Retell-purchased numbers. [(1) ]() [(2) ]() Why It Works with Retell-Purchased Numbers but Not Custom SIP Trunks For call transfers with "User's Number" as the displayed caller ID: - Retell Twilio numbers support showing the user's number on both warm and cold transfers - Retell Telnyx numbers only support this when using SIP REFER via cold transfer - Custom telephony requires your provider to support caller ID override, and if it's not supported, the transfer will fail The key issue is that Twilio requires the caller ID to be either a Twilio number you own or a number you've verified with Twilio—you cannot use arbitrary numbers as caller ID. [(3) ]() Workarounds 1. **Use Warm Transfer Instead of Cold Transfer**: Warm transfers use SIP DIAL and set the 
from
and 
P-Asserted-Identity
headers to the user's number, which may have better support depending on your configuration. [(1) ]() [(2) ]() 2. **Switch Caller ID to "Retell Agent's Number"**: This eliminates the caller ID override requirement entirely and should resolve the transfer failures. 3. **Contact Your Telephony Provider**: For custom telephony, you need to check with your provider about enabling caller ID override permissions. [(3) ]() Some users have reported that Twilio support may be able to unlock this capability for your account. 4. **Configure Twilio Header Manipulation**:
​ To learn more, see the sources I considered: [[1] Transfer call](https://docs.retellai.com/build/single-multi-prompt/transfer-call) [[2] Call Transfer Node](https://docs.retellai.com/build/conversation-flow/call-transfer-node) [[3] Issue on Warm Transfer with Users Number as Caller ID](https://discord.com/channels/1186424376300679209/1433255316186992801) [[4] Twilio](https://docs.retellai.com/deploy/twilio) ​
Feel free to tag @AI Evy with additional questions. I also appreciate feedback, it helps me improve.
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