check your BSC hardware, your AterMux interfaces, your transcoder and your A interfaces. Checking all O&M alarms is a good start, then run some diagnosis on the interfaces (if that exists ?).
If you are working with NPO (alcatel) then you can access indicators on the AIC (a interface channels), which can help you isolate which trunks are faulty.
If other vendors, it might also be possible, but i don’t know.
Well probably you should first check the cause of drops. check if there are Transcoder failures / Eq failures.. whole BSC being effected is 98% these cases.
For TRAU failures check the aters with low MHT or Ater_TS with low MHT and for EQ failures get OAN to check the Alarms and equipment.