Russell,

I am not an expert in SIP but I am quite familiar with my router. Atleast for me, the port is selected by my ATA to be in the 16384-16482 range for RTP and 5061-5062 for SIP. My router maps these to the same port numbers on the public side unless that port is being used by someone else. From what I see, the port is not selected by the media gateway.

The destination port of media gateway could be anything. But we are forwarding incoming data to the ATA. So, port number on the media gateway side does not affect port forwarding.