I have been using my U.S. Sprint SIM in one of the slots of my fairphone. It has international slow data - restricted to 2g speeds - with unlimted texts. Its the number I had exclusively growing up so it’s quite useful for talking with old friends and verifying accounts.
However, it randomly locks because it cannot connect to the network. Then I can no longer use texts, make calls, or use data. I think because the international data speed on the plan I am on is purposefully throttled, it goes below a certain level and then stops working. I have to reboot the phone or just wait a long time and it will start working again. Even more annoying is that this notification that it cannot connect to the network pops up every 30 minutes to an hour and interrupts whatever I am doing.
The other SIM - Fido in Montreal and a few SIMs I have tested out in Germany - always work and my Sprint SIM works fine if I put it into my old iPhone.
Your post suggests you are not in the US, but in which country are you? And, where do you see it “locks”? Do you mean that there is no service / only emergency calls (empty triangle icon) as SIM status?
@Lidwien, if he is roaming outside US, he won’t be affected by US frequency bands, also you can’t see the frequency in use anywhere in Android as far as I know.
By doing a web search for “the SIM card has locked” I get many hits, mainly from Sprint clients. So it doesn’t seem to be a Fairphone problem but other phones and tablets, including iphones, are also affected.
Maybe somebody with another device found a solution or workaround…