Audio stops after some minutes

Hello everybody,

I have installed about half a year asterisk(1-.4.20) with freepbx. It has a bri card with mISDN driver(1.1.7) and 5 sip users registered. Zaptel is loaded(

I did not have some serious problem with asterisk since last week.I realized that when I speak(sip user) with some user (via bri card) more than about 15 minutes audio stops and I cannot hear the other and also the other person cannot hear me.We end the call-hang up the line, and call again. This does not happen to all calls but to most…

I use hardware phones-linksys(942)