There are two reasons your SIM could be locked by FPLMN
- If the operation is performed while the module is scanning/attaching to a network it can interrupt and restart that process on some modules (resulting in unnecessary over-signalling).
- If the firmware clears this list frequently and is spinning in a loop then that can result in excessive signalling (through excessive retries to the same operator).
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.
To clear the FPLMN you'll first want to consult your modem's documentation to make sure you are aware of any special instructions as usually, you'll need to set it to minimal operation mode or airplane mode, depending upon the module. In this example, we are setting the device to minimum functionality which disables transmit and receive RF circuits for both circuit-switched and packet-switched services with at+cops=0.
Clear the FPLMN:
be sure to include exactly 24 "F"s.
Last, don't forget to switch your device back to full functionality if that was modified.