### Answer Based on the documentation, dynamic variables can be used in the transfer call tool's "transfer to" field [(1) ]() [(2) ]() , but the sources don't explicitly confirm whether they work in the extension field specifically. The documentation states that dynamic variables work in "Transfer call phone numbers" [(1) ]() , and you can set the transfer number to be "a number in e.164 format, or a SIP URI in the format of

sip:username@domain

(e.g.

sip:user@retellai.com

) " or a "dynamic variable that gets substituted at runtime" [(2) ]() . For the extension field, the documentation notes: "Set the transfer number extension if needed. Extension must be 0-9, '*', '#' (E.g. 123#) " . However, there's no explicit mention of dynamic variable support for the extension field. Given that dynamic variables are supported in the phone number field itself [(1) ]() [(2) ]() , you might want to test whether they work in the extension field or reach out to support for confirmation on this specific use case.

​ To learn more, see the sources I considered: [[1] Dynamic Variables](https://docs.retellai.com/build/dynamic-variables) [[2] Transfer call](https://docs.retellai.com/build/single-multi-prompt/transfer-call) ​

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