Mobile gets LAU (Location Area Update) reject with cause PLMN not allowed
There are many scenarios by which Locarion Area Update(LAU) will fail. One such scenario is when network will send message to the mobile station(MS) mentioning LAU reject with cause as PLMN not allowed.
This is one of the simplest reject cause which sometimes engineers need to analyze either in the lab or testing the DUT in the field. The DUT can have one RAT(Radio Access Technology) or multiple RATs stack running on the chip. This DUT is referred as multi mode modem.
This situation can arise when mobile station subscribed to PLMN say 'A' tries to access the PLMN say 'B' for which it is not subscribed. In this scenario network response to location update request from mobile will be with cause PLMN not allowed.
In this scenario mobile station is not allowed to access any part of PLMN 'B'. MS need to manually or automatically search for other PLMNs in the coverage area of the mobile.
This is the simplest case when MS is subscribed to say verizon wireless and trying to register with T-mobile. Also there should not be any aggrement between both the networks. Here T-mobile network will reject the Location Area Update (LAU) request with cause as "PLMN not allowed".