Incoming calls problem


I have a problem with the incoming calls after been registred about 60 seconds.
After this ~60 secondes I can get any incoming calls.
When I use the field qualify = ‘yes’, I see that after this 60 s my phone is UNREACHABLE. But when I set 60 sec in the value of the expire time on my phone (Swissvoice Ip10s or the Thomson ST2030) my phone is always rechable and all works fine.
My question is what I have missed and what I have to do for using the default value of the expire time (3600 s) and make my phone every time rechable.

But what is strange, I have this probleme in a thomson voice and swissvoice ip10s but never on the softphone sjPhone for instance.

Thanks in advance for your answer.