I am having half channel audio after about 6 minutes. asterisk 1.2.11 and 1.2.8 zaptel. My call is originating on a a SIP extension going out to nufone to my cell. After about 6 minutes the SIP extension can still hear me on my cell but I cannot hear the SIP extension. If the SIP extension hits a key I hear it on my cell. If the SIP extension is talking and hits hold the cell hears about 2 seconds of audio then nothing again. What might this be or what should I do to find out what is going on. Jerry