asterisk users - Nov 2003 - Incoming calls randomly hangup and blank calls

Hi,

I have little problem and it is so embracing when u r talking to some one
and line get hang-up. 
When some one calls from out of state or out of country my calls gets
randomly hang-up with in few seconds and it happens with most of the calls.
It's happening randomly I got few calls, which worked. 

I have also observer that with quite few call I can't hear the person and
person can hear me. If they try to leave the voicemail and this problem
occurs then, voicemail will hang-up on user, because it think there is no
voice, so we get half voicemail of that person or blank voicemail. 

I don't know weather this two thinks are related to each other problems or I
have two different problems.

I have removed busydetect=yes and callprogress=yes then also same thing,
tried to set busycount=6 but no luck. Has any one facing this kind of
problem in *

My setup is DID inbound line, incoming only, to  Adtran TA750
(FXS-CARD)----connecting to * on T1 with t100p card. 

Outgoing calls are working fine, since it going through normal CO lines and
FXO card.

Can somebody put there input, if they had faced these problems.

Any help is appreciated.

Regards,
Kekin