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.