I am posting this in the hopes that it gets to the top of Google, because at the moment, the results are not very good.
Call this number:
0330 123 4151
when it says ' what are you calling about ' - say ' changes to my account '
it will ask if you meant ' do you want to change your account ' - say yes
It will then ask for your phone number (which I never know in the first place - who knows their bloody land line number these days?) - if you ignore it completely, it will ask about 4 times before diverting you to a human.
BT's system is AWFUL! and the website is even worse (i had to call them to change billing details, even though this page: http://www.shop.bt.com/help/managing-your-account/how-can-i-add-or-change-a-payment-method-on-my-account/ says you can do it online (it contains dead links))