If the call is getting muted/blanked in a particular BSC area, irrespective of the BTS, then the problem could be narrowed down to the transcoder cabling/CIC matching.
It it is getting blanked under certain BTS only, the problem is more difficult to solve and you need to first investigate with 1 BTS first. The most common problem I have faced is that the mapping of TS in Nokia BTS S/W(i forgot the name) could be difft from one that loaded in the BSC.
Check these 2, and if u still find blank calls, then go for a complete freq remapping in the whole n/w!! just kidding!!