Do you have longer SDCCH MHT?
We have some phones in our network but still cannot find them because in the ass message for LU there is no info. These phones loose network when crossing LA and continuously try to do a LU – one phone is ~ 1200 attempts per hour. We discover that this is true for a pairs of sectors lets say. Because after 4 unsuccesful attempts the mobile performs forced cell reselection and again 4 att and back…
We have great difference between succsesful seizures of SDCCH with reason LU and succesful assignment of SDCCH with reason LU. SDCCh is released after a timer expires and this is counted as traffic although noone is connected.
Also check routing area update.