You can easily cancel 1571 online. Go to www.bt.com/callingfeatures and click the Manage my calling features button. This will start an online order process. Put in your home number and postcode and confirm you're the BT account holder to continue.
The next screen will show any calling features that are currently on your line. To cancel 1571, scroll down and click the "Remove all my messaging features" radio button. Alternatively, if you'd like to keep BT Call Protect, click that radio button instead.
Click Continue to complete the cancellation order.
You can also manage all your calling features (including ordering, upgrading and cancelling them) at www.bt.com/mybt >