i was told by one of the nokia trainers that if u have halfrate enabled in the cells then the chances of getting this alarm is high, thats because during the peak hours the halfrate transcoders could be completly full and hence that half rate call can be dropped due to transcoder pool cong. So u need to work on the half rate transcoder pool and check if there is any congestion.This could be one of the reasons apart from what has been mentioned already.