There is a request from Asterisk to Twilio and the response from Twilio to Asterisk. It is the latter that has the 5060.
Addition: The REFER is being sent the URI specified in the Contact header provided by Twilio, which is correct, from the point of view of Asterisk, although questionable for Twilio. It is being sent over the existing TLS connection.
I think you need to specify the parameter of Transfer as PJSIP/sip:[email protected] but there may be subtleties. The documentation suggests that the PJSIP/ is optional