The following headers will always be added by Twilio to the outgoing initial INVITE request:
In order to better integrate with remote SIP applications, Twilio's SIP connections can read headers that are returned in the response messages. Twilio will read any headers beginning with the X- prefix and copy them to either the Dial action URL or the call screening URL, in the format DialSipHeader_X-headername=headervalue.
On a successful call setup (when a 200 OK SIP response is returned) any X-headers on the 200 OK message are posted to the call screening URL. Headers on the final SIP response message (any 4xx or 5xx message or the final BYE/200) are posted to the Dial action URL.