RTP traffic is transfer to IP Phone A to IP Phone B so Asterisk is not in the middle
In this case: canreinvite=yes, but it's not the only one reason.
One day, i had seen these parameters into an old distro (Resix under Asterisk 1.2
), and all extensions had these parameters for each one. Maybe it was false, sure.
rx/txgain should be included for every hardware devices configuration, like FXO, FXS, ISDN, or else.
SIP, IAX2, H323....: none.