every t3212, the MS autonomously sends a LU.
the network does not ask for anything.
after a certain timer “t detach”, the VLR will detach the MS from the network, if no LU were received.
if t3212 = 6hours, then t detach = 13hours for example.
when there is a call, the Ms is paged normally by the network. At the end of the call, if t3212 expired during the call (how unlucky !), then a LU is performed immediately after the call.