I am working on CCM 5.0 and Cisco IP Phone 7961. I have a problem in communication btwn IP Phones?

I am working on CCM 5.0 and Cisco IP Phone 7961. I have a problem at one location that when my enduser place a call to particular IP Phone and when call is connected,end user can hear my voice but i cannot hear the voice of end user. Here is nothing related to Mute on. This problem occurs in 2 locations that is of same District and all network IP Phones going good. Please suggest something what is wrong here...