Audio stops after x seconds

Hello all,

I have installed asterisk with a bri card about 7 months. Until now I did not have any problem. Around a week now, I have the below problem:when i speak about 15 minutes and more i can not hear the other, and the other person cannot hear me…so we hang up and call again

asterisk version:1-4.20
zaptel 1.4.10.1
mISDN 1.1.7

the zapata.conf and misdn.conf have the default settings.

Can you help me??? thanks