it might be a sector inversion, or a hardware failure from a TRE or ANC (combiner), or a hardware failure from the transcoder,
You ought to perform some detailed Abis traces :
when does the call stops ? (AGCH, SDCCH, TCH phases ?)
on which TRE are you on ?
does it drop on all TRE ?
does it drop on the other sectors too ?
are the other BTS on the same Abis link impacted too ?
on the whole transcoder ?
on all the BTS of the same transcoder’s TCU ?
I hope it can help… those are just possibilities. Try also to go in the cell with an handover, instead of a call setup. And check out your QoS indicators, what are they telling you ?