Problem when SIM in slot #2 is configured for 4G (the one in slot #1 is 2G)


in order to make use of different data plans, I regularly use both of my SIM cards for mobile data (of course only one at a time). The inactive one is always set to 2G explicitly before enabling 4G on the active one.

When the SIM card in the second slot is selected for mobile data (regardless of mobile data enabled / disabled), the modem frequently “crashes”: I have to re-enter the PINs of the two SIM cards. After that, telephony is back again, but I won’t get back mobile data unless rebooting the phone.

Initially I thought that this is a bug in the /e/ operating system (which I am using). But then I got a report of the same symptoms by a FP2 user who runs LineageOS 18.1.

So my question: does anyone see similar symptoms with FairphoneOS?

Does this look like a software issue (for instance in some low-level drivers which are shared between different operating systems)? Or is this more likely a design glitch in the FP2 electronics?

More details to be found in my report in the /e/ issue tracker.

Kind regards

Yes, I experienced this issue during the warranty period and it fixed by a core module replacement. It’s possible that the problem is not linked to /e/ os.

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Please tell us in which country you are and from which operators you got your SIM cards.

I haven’t used dual SIM myself for a few years so I can’t tell whether this is also an issue in recent versions of FP OS.

German SIM cards, both used in Telefonica/O2/Eplus network (sipgate / 1&1). To quote from the ticket in the /e/ issue tracker:

There are two SIM cards, both report MCC=262; MNC is 03 and 07, respectively.


Probably I miss something, but when you select a SIM for mobile data, mobile data are enabled. So where do you select.without enabling? I use 2 Telefonica SIM since years, however normally only 1 for mobile data and have no issues when I switch mobile data between the SIM slots. I use LOS 18.1. Have you interchanged the SIM cards to see if it is a SIM card failure?

but when you select a SIM for mobile data, mobile data are enabled

With “select for mobile data” I mean the configuration in the Settings. When you turn on mobile data for a certain SIM card this way, that card will become the default for later quick changes of enabled / disabled mobile data by pulling down the status bar and tapping the symbol for mobile data. In the problematic case, the SIM card in the second slot is set as “default for mobile” data, but it is irrelevant whether mobile data itself is enabled or disabled.

Have you interchanged the SIM cards to see if it is a SIM card failure?

Yes, swapping the cards has no impact. It is always the card in the second slot (when “default for mobile data” and configured for 4G) which leads to the problem.

Please note that I suspect changes in the mobile cell environment as potential trigger. Fir instance moving around or spontaneous downgrades to 2G by the tower.


@sebbaz, do you happen to know whether the root cause had been identified?

@yvmuell, did this clarify what I had been trying to describe?

@ingolf yes it was clear with this explanation, however unfortunately I have no real idea why this happens.
when exactly does the system crash, only at the moment when you switch to the SIM in slot 2 or frequently while it is selected (and how often)? Does mobile data in slot 2 work (as long as the system does not crash)? Before you change mobile data to slot 2, do you set slot 1 to 2G? With LOS 18.1 (and e/OS is based on LOS), its possible to have both slots set to 4G, however, this should not be possible as the FP2 can normally have only one SIM set to 4G, the other SIM has to be set to 2G

@yvmuell, the crashes occur at a later time. For some time after switching to the SIM card in the second slot, I can successfully use mobile data via that card (with the expected performance).

Later, at some unspecified point in time*, the modem will “crash” (I cannot tell whether indeed some process crashes), and the PINs need to be reentered. It is definitively not a reboot, as the PIN entry overlay appears spontaneously without any previous non-nominal symptoms. After I have re-entered the PINs, telephony is back again, but mobile data won’t work unless I reboot the phone.

Yes, I explicitly set the card in the first slot to 2G before configuring 4G for the card in the second slot.

* I suspect that the trigger might be a change in the mobile cell environment like change of tower or (tower initiated) change between 4G and 2G. The reason is that sometimes (when the phone is not moved significantly) I will take a few hours before the “crash”; at other times, I can pretty well predict that it will “crash” soon because I am moving.

Kind regards

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