High port SIP registration

My asterisk box is behind an isa2004 box and it registers fine with my sip providers. The only problem is that sometimes the calls get through an other times my sip provider cant get the call to my box. They have check and it seems that asterisk is registering with a high port number e.g. 53598 and a different port is used on each registration.

Everything in asterisk is set to use the standard port and I have no idea where theses random port number are being chosen from.

Im guessing its something to do with my isa box.

Any help would be appreciated.

asterisk will REGISTER and send sip commands over udp port 5060 (unless you tell it otherwise). RTP traffic (voice media) is sent on a high range of ports, by default 10000-20000 but that can be changed in RTP.conf.

If your SIP registrations are coming from 50k range ports, your ISA box is screwing it up…