This suggests that the ATA is setting a session expiry in the initial INVITE less than 15 mins and so refresh messages (RE-INVITE's) happen soon enough to keep the NAT port open, whereas Callcentric have a much longer expiry on their invite.
What I don't understand is why the ATA NAT keep-alive's are not doing their job.
Another possible option is to reduce the REGISTER timer from the default 3600 seconds to 60 seconds. ITSP's don't like this as it increases load on their system if everyone did this, but just you doing it won't be a problem.
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Surprisingly enough Callcentric actually recommends to set "REGISTER EXPIRES" to 60 in their setup guides, and just double checked my own set up and yep, it's set to 60 on the Callcentric line.
I guess I'm not entirely clear on how the Asus DMZ function works because I figured that would eliminate any kind of port forwarding, NAT problem as being the cause. Is that not right?