[Serusers] call drops after about 30 seconds

hello everyone, I am using ser-0.9.0 together wit asterisk-1.0.6. I am testing it with a couple of hardware AT-320 IP phones from Atcom, using a PA168S chip. They seem to be working fine, with stun behind nat, and they can call other user-agents (softphones, asterisk adn the pstn through it), but whenever I make a call between the two phones, everytime the call is dropped after about 34 seconds, even when calling between phones on Public IP addresses. I am actually clueless why this happens, I tried changing the NAT ttl value, bu to no effect, and it is not supposed to be a codec mismatch too since both phones use absolutely the same codecs, in the same order of preference. Appreciate the feedback, cheers -- I'm sick and tired of being sick and tired...