Intrstg! Call drop in 10 min.: ICR

In circle roaming is on in a circle i.e. 2 operators A & B sharing the same BTS. Right in the middle of the shared LAC (i.e. not on LAC boundary) calls are being tested using Post paid SIM.
In one of the operator, say A, Call gets drpoped every 10th minute like at 10 Min 43 sec. or 10 min 50 sec. ……while at same time & same location other operator’s, B, SIM is still on call! I suspect a MSC Timer issue.
In MSC, PLMN with IMSI are defined that seperate the two Operators subscribers. Billing is obviously done seperately. Any suggestions guys??