My phone appears to not ring every time. Occasionally it does ring but mostly not.

I have tried to turn off call waiting or even see if it is wrong but with no joy.

I bought this abroad (e-king) and wondered if it was because I had put firmware on the mobile and locked it back up. Don't know much about the locking/unlocking but I would imagine it is not hard to do. Just need to know if this is the cause.