The telephone number 0800 800 150 belongs to British Telecom and can be used to inform BT that you are moving and want to close your account.
If you have received a letter stating that ‘Someone wants to take over your line,’ then you should phone BT as soon as possible. Several UK citizens have complained that they have received a letter or text suggesting that someone has applied to take over their line. If you receive an unexpected letter or text about this then treat it with caution and phone BT back to confirm that you are not moving, that you would like to keep your line.