Hello Pix:
You said that there is a timer in the NSS that can expire. If the NSS receives a LU before the expiry, then the timer is restarted. Otherwise, the mobile becomes “IMSI DETACH”-ed. So ensure that your NSS timer is well defined.
Please, I urge to know more details about this timer. I have a problem of Paging Failure, and it might be due to this NSS timer!