yes, the timers are fine. I’m not familiar with the guard timer (GTDM) though.
It seems quite radical to have T3212 = BTDM. I would allow few more minutes for the BTDM. But anyway, that’s not the problem here.
so let’s focus somewhere else.
perhaps it is due to bad coverage in some areas.. or unstable BTS that provides coverage only once in a while.
before the T3212 expires, the MS is actually ALREADY out of coverage and cannot be reached. During this time, the MSC still has the Ms attached, but physically the Ms is not reachable.
however your timers are already quite short (108mn), which is 3 times less than standard duration. The benefit is that such “unreachable” situation might last ( in the worst case ) 107mn max.
in order to verify this hypothesis, you could try to decrease T3212 and BTDM. If you see an improvement, then… good.
also, do what Smart asked : trace the IMSIs which are unanswered. If they are always the same, then… that gives some pointers.