this problem is going to be counted as a call drop in the BSS (= TCH drop). do you see a huge amount of drops in those cells ?
do you see a radio problem in your drivetest ?
i can guess your answers, so at least it proves that it really looks like a BSS/NSS misunderstanding… have you tried performing some traces on the A interface ? do you see the assignment complete ? do you see the alerting ?
could you check with NSS engineer if they could trace the issue within the MSC ?