LAU reject with cause Roaming Not Allowed
In this scenario is when network will send message to the mobile station(MS) mentioning LAU reject with cause as Roaming Not Allowed.
Mobile subscriber(MS) receives LAU reject with this cause from network usually in VPLMN (visited PLMN) when MS can access only part of VPLMN and the other part is not accessible. In this situation, MS will try some other PLMN in the same area of VPLMN.
This cause basically indicates to the MS that "this PLMN in this area is not for you, try some other PLMN". AS said, this cause is always sent by VPLMN and not HPLMN because HPLMN always is designed to serve the subscribed MS.
For explaination let us assume that PLMN operator has GSM networks of verizon wireless and T-Mobile as shown. The PLMN operator Verizon has roaming aggrement with the operator T-Mobile for all the LAs except LA2. Hence subscribers of Verizon can roam in any of the location areas of operator T-Mobile except LA2.
If a subscriber of operator Verizon tries to register in the area LA2 of T-Mobile network, then the network T-Mobile will transmit the message LAU reject to MS with reject cause as roaming not allowed in this location area.