Voice quality issue

The main office is in Yalta, and I am in Lvov. The operators are calling on the mobile phones through the Portech gateway. All the equipment is placed in one the same office. It sometime happens that the operator doesn’t hear the voice on the other end. What is the problem in? Please advice.

P.S.: On the records (with this proplem) everything is good, everything is clearly heard. Can not monitor it, as the problem happens from time to time.

The phones used for calling are Cisco 7940.