asterisk users - Apr 2006 - Problem with Voice quality, please help

Hi All,

        We made a VOIP application for PDA's (PALM OS) and we are using both
SER
and Asterisk. SER is SIP proxy and it routes all the calls to Asterisk. On SER
we have RTPProxy also. My problem is that I am getting a weird noise or
disturbance for all the calls at an approximate time interval of 100-120
seconds and we are getting this noise consistently. After 5-10 seconds
everything will be fine till next 100-120 seconds. We are using GSM codec on
client side with GSM voip provider. Can you please tell me where it is going
wrong and where can I change or improve things for better voice quality? Is
there any echo which is causing this?

Thanks,
Manoj.