if there is an alerting message in DL for MS1, it means the MS2 is ringing… are you sure that the phone number you are calling is the MS2 ?
If yes, then it means the MSC is actually answering the phone instead of the MS2. So that corresponds to a call being diverted to the voice mail directly. That could be a special setting of MS2, or just that the MS2 cannot be paged by MSC.
I doubt that MS2 would miss the paging because of cell reselection ! The pagings are repeated by MSC, to avoid such case.
Is MS2 IMSI-attached ?