​ To learn more, see the sources I considered: [[1] Transfer call tool (single & multi-prompt agents)](https://docs.retellai.com/build/single-multi-prompt/transfer-call) [[2] Transfer call failure: “probably don’t have the permission to perform the transfer…”](https://discord.com/channels/1186424376300679209/1450603538798608436) [[3] Discord message from @johnwicked33_ on 2025-10-30](https://discord.com/channels/1186424376300679209/1186462578310910112/1433402001106014219) ​

14219) . **Suggested fix based on the documentation**: switch Displayed Caller ID to Retell Agent's Number, or confirm your telephony provider (e.g., Twilio vs. Telnyx) and transfer type (cold vs. warm) support caller ID override for the User's Number option [(1) ]() . If this doesn't resolve it, I don't have further documented root-cause information in these sources. Want a general answer based on my broader knowledge instead?

### Answer The official documentation doesn't state an explicit root cause for that specific error message. Here's what's directly relevant: This error is tied to caller ID override during transfers. When you set the transfer to display the User's Number instead of the Retell Agent's Number, that requires your telephony provider to support caller ID override — and if it isn't supported, the transfer fails [(1) ]() : - Warm transfer uses SIP DIAL, setting the

from

and

P-Asserted-Identity

headers to the user's number - Cold transfer uses SIP REFER; supporting caller ID override is up to the telephony provider - Retell Twilio numbers support showing the user's number on both warm and cold transfer; Retell Telnyx numbers support it only via cold transfer (SIP REFER) - If caller ID override isn't supported, the transfer fails There's a documented FAQ entry on this exact symptom: "The transfer destination sees the wrong caller ID. What's wrong?" — Showing the caller's number requires caller ID override support from your telephony provider. If override isn't supported, the transfer fails. The suggested fix is to switch Displayed Caller ID to Retell Agent's Number or use a supported number . Separately, in a Discord forum thread, a community member reported the same error message intermittently on warm transfers and was advised to switch Displayed Caller ID to "Retell Agent's Number" instead of "User's Number," though the root cause wasn't confirmed in that thread [(2) ]() . Another Discord message describes the same error occurring on a warm transfer using SIP trunking (Twilio) while trying to keep the user's number as caller ID [(3) ](

Feel free to tag @AI Evy with additional questions. I also appreciate feedback, it helps me improve.