Ran,
There must be a problem in the quality or the signal. There is a call drop if the BTS doesn’t receive a radio signal from the mobile for about 8 seconds (depending on your parameters settings).
Check the Radio Link Time Out value, and if it is set to > 16 SACCH.
Check also the cause of the TCH drop (radio ? handover ? internal failure ? transmission (abis) ? … etc … )