### Answer When buying or importing a phone number, TCP and UDP refer to the outbound transport protocol used for SIP signaling. [(1) ]() - **TCP (Transmission Control Protocol) **: Connection-oriented, ensures reliable delivery of data. This is the default transport method in Retell. - **UDP (User Datagram Protocol) **: Connectionless, faster but less reliable. The key consideration is **compatibility with your telephony provider**: - Some providers only accept UDP (e.g., certain Asterisk setups) . [(2) ]() - Telnyx documentation recommends selecting TCP as the transport method. [(3) ]() If your provider only supports UDP, you'll need to select UDP when connecting the phone number—otherwise calls may fail to route properly. [(4) ]() Valid values for the

transport

field are

"TLS"

,

"TCP"

, and

"UDP"

. Default is

"TCP"

. [(1) ]()

​ To learn more, see the sources I considered: [[1] Update Phone Number](https://docs.retellai.com/api-references/update-phone-number) [[2] Discord message from @boroca.guy_67514 on 2025-11-07](https://discord.com/channels/1186424376300679209/1186462578310910112/1436417821755310341) [[3] Telnyx](https://docs.retellai.com/deploy/telnyx) [[4] Our outbound calls are broked after last update](https://discord.com/channels/1186424376300679209/1439873143832580117) ​

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

### Answer For Twilio, Retell supports the following transport methods: TCP, UDP, and TLS. [(1) ]() When configuring transport for inbound calls via SIP, you can specify the protocol by appending it to the SIP server URL: - TCP:

sip:sip.retellai.com;transport=tcp

- UDP:

sip:sip.retellai.com;transport=udp

- TLS:

sip:sip.retellai.com;transport=tls

For outbound calls, when importing a phone number, the

transport

field accepts

"TLS"

,

"TCP"

, or

"UDP"

, with

"TCP"

as the default. [(2) ]() Retell also supports SRTP (media encryption) , but transport should be set to TLS to use SRTP. [(1) ]() For detailed Twilio SIP trunk setup instructions, check out the [Twilio guide]() .

​ 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) ​

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