With BT, the cut-off time (for relevant calling plans) is 7pm these days, not 6pm. Anyone using their 'Unlimited Evening & Weekend Calls' plan gets free calls between 7pm and 7am (and from 7pm Friday to 7am Monday) to 01, 02 or 03 numbers.
However many people will be on their 'Unlimited Anytime Calls' package, always giving them free 01, 02 and 03 calls.
Only a small minority of people still use their basic 'Pay as you go' plan, where calls are charged at the same rate at any time of day or night.