I had a problem for a while whereby, after I switched to Jolla (which required cutting my SIM card), I could not switch back to my N900. The phone asked for the SIM card PIN but that's it, no mobile signal after that.

To cut it short, the problem was that I somehow switched my N900 to "2G only", which rendered the phone incapable to even register with my mobile network (3 UK). As the name suggests, the network does not even have the 2G infrastructure, it joined the scene after the introduction of 3G. Switching the phone to "2/3G" or "3G only" saved the day.

How would you explain that? The phone needs 3G at least for the registration, but then uses 2G for making calls? Or are the calls somehow routed through 3G after all?