For the SIP Registrar feature to work correctly, select UDP at either the IPSP or TG level, for the REGISTRAR TG.
At the SIP Sig port level, if none of the sig ports in the zone have UDP supported, the CLI validation returns a failure while setting the Registrar flag. If the REGISTER flag is enabled without the recommended configuration setting, the REGISTRAR rejects the message with a 500 Internal Server Error.