According to BT, 0870 numbers are Nationalcall rate numbers (not premium rate) where users pay the long distance rate regardless of where they are calling from.
Subscribers do not receive a percentage of each call charge, but they do receive payment for call volumes in excess of 10,000 calls a month.
Call charges vary from 1.5p per min at weekends, to 3.75p per min weekday evenings and nights, to 7.51p per min weekday daytime (incl VAT).
BT say that users often mistakenly confuse charges for 0870 numbers with those for 0871 numbers, or some of the more expensive premium rate services, which have a fixed rate at all times of day and also provide subscribers with a share of call generated revenue.
David