This failure is VERY strange, it’s very difficult to identify the problem.
Does it happen on all cells of the BSC? Or only on cells with external neighbours ? (from other BSC)
Try to reduce the size of TCH queues (T11 less than 6 seconds, BTS Q LENGTH less than 5), try to disable Forced Directed Retry (EN FDR = Disable), try to disable Fast Traffic HO (En_Fast_Traffic_HO = disable), try to forbid external directed retry (EN_EXT_DR = disable). Attempt those actions only one by one. Don’t rush 🙂 The External Directed Retry and Queuing could generate BSS errors if it is not understood by MSC correctly.
If you still have failures, then it’s perhaps a discrepancy between NSS and BSS. Check that your NSS have the latest patches… (that happened to me… NSS update fixed the problem)
Also, check BSC Radio parameters are in line with MSC configuration. For example, below, there are some parameters set in BSC that must be checked by MSC team, so that both sides are using the same timers, same messages, etc.
T(iar) & T(ias)
T11 & T11_FORCED (T11 forced Must be < assignment timer in the MSC (trr1)) T9110 etc...