Is there any way to unlock my phone from a mobile network carrier so I can use a different sim? I've got a BQ Aquaris M5. BQ don't have a telephone number I can ring or much of a support base, so I wasn't able to find help there.

I bought the phone sim free and put a giffgaff sim in it. Apparently giffgaff don't usually lock the phone to the parent O2 network, but this can happen as a bug on some iPhone models - I think the same thing has happened to my BQ Aquaris M5.

Recently I bought a replacement three sim on a contract - I tried putting this sim in the phone but it just says "emergency calls only". I rang three to get technical support with this but they weren't able to do anything at their end, and suggested to me that my phone might be locked to some other network.

I'm wondering if it is possible to "hack" (loose use of this expression) the android system in some way to unlock my phone from a particular network? Alternatively, perhaps this data isn't stored on my phone but is stored remotely, and therefore this isn't possible?

Either way it would be useful to know.

I have tried searching around google, but so far I can only find info on PAC / PUK codes, which I don't think is what I am looking for?

I have found my IMEI code - not sure if that is useful?
I can't believe it but I managed to resolve the problem.

Just a quick list of details to help future google searches:

I had a BQ Aquaris M5. Bought it sim-free, but then added a giffgaff sim.

Later I changed to a three sim, removed giffgaff sim and inserted three sim, but three network was not recognised. (Emergency call only.)

Resolved problems by going to System Settings -> Data Usage (or show more) -> Cellular Networks (might be hidden in top right corner) -> Set Preferred Network type to 2G/3G/4G AUTO, also go to Access Point Names, click Reset in right hand top corner menu, then go to Network Operators and check network operator is correct. Possibly restart your phone.

Happy to report mine is now working!