onCallTsxState

Transaction-level callback. We use it to inspect the raw incoming SIP messages for a CANCEL carrying an RFC 3326 "Reason: SIP;cause=200" header, which signals the call was answered on another device (shared/forked call appearance). PJSIP does not expose this header through CallInfo, so reading the raw message here is the only way to tell it apart from a genuine missed call (both terminate the INVITE with 487 Request Terminated).

Heavily guarded: not every transaction event carries an incoming message, the underlying objects are short-lived, and a failure here must never disrupt normal call handling.