Have you looked at the messages logged in the drive test (layer 3)
– what happens just before the drop ?
– what about qoS statistics in serving and target cells ? do you notice something in terms of handover failures or call drops ?
i would suspect that the two location areas are located in different MSC’s, right ?
you could check neighbour definition and LAC definition in the different MSCs (can’t tell you more than this, you must check with NSS engineers)
you can also ensure that both cells are using the same ciphering algorithms, as it can lead to a drop when switching ciphering in dedicated mode (as I experienced, but it might not be true with every vendor !)
if the cells are from same MSC, but different BSC, then there is probably something you need to investigate on the A interface.
Altogether, you can investigate further with detailed QoS indicators and/or A interface traces, and simply have a look to your layer 3 messages in the drive test.