In four words: don't go with them.
Steve hasn't used the number for literally years but has been paying for it the whole time.
He cancelled his account last month and they said there was a final £30 charge for cancelling which Steve paid and was told that was it... no more payments, the account was closed.
3 have just been on the landline phone again, asking for another £30 because of the 30 day cooling off period.
So Steve's paid an extra £60 on top of the contract he'd been paying and not using the number for years and years.
He hadn't even used the number for so long that he doesn't know what the phone number on the account was!
If you've got an account with 3 and you're happy with it, that's fine... just expect to pay through the nose for cancelling the contract with them!