I have solved it for my M250K.
Confirm your phone doesn't recognize "my phone number"
Settings -> About Phone -> Status -> My phone number
likely, it says "unknown". I dont think there is a way to change it in your phone so you need to put your SIM in another phone (such as iphone 3G/3GS, or older Sony Ericssons) and edit it there. For iphone 3GS, you can do this:
Goto Settings> Phone> Press My Number> Enter YOUR number> Press Save> Press Home
This will save to the SIM. When you put the SIM back in, you should be able to use it.