asterisk users - Aug 2004 - zap channels and sip channels problem

Hello,

I have installed 4 port FXO card.
I have a problem when I am calling from SIP device to ZAP channel (calling
to outside line)

First couple of seconds calls is normal but when I stop speaking to
microphone for a couple of seconds (so for a couple of seconds it is
silence) then when I start talking again the other person do not hear me
anymore. But I can hear the person on the ZAP channel.

What could be the problem ?

Bart