A quick search around the internet I found this click here
So it's not really new for BT to charge for the 1571 service because part of that says:
* If you are billed monthly, at least two chargeable or inclusive calls must be made with BT within that month. Otherwise a charge of £1.00(Inc VAT) a month for the BT Answer 1571 standard service will be raised.
* If you are billed every three months, at least six chargeable or inclusive calls must be made with BT within those three months. Otherwise a charge of £3.00(Inc VAT) every three months for the BT Answer 1571 standard service will be raised.
The worrying part is being billed if you use another network over BT lines and don't dial 1280 because if you are not a BT customer, how are you going to know about the charge unless your own provider informs you. The other question is that two calls each and every month, or two calls to switch the system off? If it's the former, then in sounds like a nice little earner for BT if you forget to do that. If I've understood that correctly then I'm not sure they can do that without being challenged because the onus is now placed on a subscriber to op out of a service rather than op in. Having not seen the letter then it's difficult to determine what they are actually saying.