The FPLMN list (aka forbidden network list) is stored on a SIM and tells a device which networks it can’t connect to.
The forbidden list reduces attempts to connect to networks that aren’t available, but sometimes it needs to be cleared when debugging.
How does the FPLMN list work?
FPLMN lists are helpful because they tell a device which networks it shouldn’t attempt to connect to. This shortens the time it takes to connect because the device won’t look for networks that aren’t available.
The FPLMN list is a dynamic list that is stored on the SIM, but maintained by the device’s cellular module.
When a module attempts to connect to a network and is denied, it will write that network to the FPLMN list on the SIM, thus preventing it from attempting to connect to that network again.
How can the FPLMN list cause problems?
FPLMN can sometimes cause issues, effectively preventing your device from accessing local networks.
For example, if a device is turned on when a SIM is deactivated or if a device tries to connect to networks not included in the Network List assigned to the SIM (aka "whitelist"), the device will write all local networks it tries to connect to onto the FPLMN list (aka "blacklist").
We recommend implementing some firmware logic that will force the module to clear FPLMN automatically, either periodically or after every X unsuccessful connection attempts. Consult your device manufacturer for further instructions on how to set it up within your specific module.
If any networks have been blacklisted or if the SIM has since been activated and you are still not seeing successful connections, you should consider clearing the FPLMN list.
This is because the device will not attempt to connect to networks in the FPLMN list while the SIM is in automatic network selection mode. You can also read more on how devices use PLMN lists on the SIM during automatic selection.
If you suspect that your device may have written some networks onto FPLMN, you can clear the list by following this guide: How to clear the FPLMN list on the SIM.