vicky, i would appreciate a little more details.
what do you see during drivetest, will there is “blank” in the voice ??
you don’t see anything different ?
then it means the TCH are correctly received, but they contain no voice.
Either the TRX receives voice from TC but due to malfunction, there is no data encoded/modulated in the radio timeslot.
Or the speeh frame received by the TRX is already empty. In this case, the TC is faulty. If it happens randomly on many cells, and randomly during the call, then it’s probably a temporary failure of a board.
Could it be connected to DTX (DL or UL) or specific speech codecs (AMR NB, AMR WB ?), or tandem free operation (TFO). All these features require some process within the TC.
try disabling these features for a while, just to be sure.