BT have decided to change the time of "evening" calls.
Evening will now begin at 7pm NOT 6pm.
Therefore, if you are on one of the anytime tariffs which has free evening calls, you will pay for calls between 6pm and 7pm. Not only that, but these calls will cost you more than than they currently do as BT are charging more to "set up" non-inclusive calls.
It's on the last page of the booklet they may have sent you recently.