Changing the number on a S/H mobile phone

[ppppenguin]

If you then try to use a locked phone with a SIM car for a different network, it won't work - you have to get the phone "unlocked" first. This may or may not be a legal thing to do, as explained above.

Just to clarify. Unlocking a phone that locked to a specific network is always legal. A phone that's barred won't work on any networks and is effectively a brick. I'm sure there are illegal methods to unbar a phone but GVR is not the place to discuss them.