The FPLMN (Forbidden Public Land Mobile Network) list is a collection of data stored on your SIM that consists of forbidden networks. Traditionally, modems append networks to this list after failed attempts at registering onto them in order to speed up connections in the future. In order to determine if you have anything written on your FPLMN we recommend you run the following AT command:
Here we are telling the SIM to give us datafile 28539 in binary. Notice from the picture below that we get 24 "F"s this lets us know that the FPLMN is empty which is what we want. If you have anything other than 24 "F"s then something has been written onto your FPLMN.
Clearing the FPLMN is usually something modems do on their own, either on reboot or after certain conditions. However, if you prefer to do it yourself you simply need to send the following AT commands: (please note that manual clearing is for one off situations and should not be automated as that will cause issues on the network)
be sure to include exactly 24 "F"s.