0871 numbers have two parts to the price of the call.
1. the revenue share part which costs up to 10 p per minute; this money is passed on from your phone company to the other phone company. Whether they then pass it on to the company you called or just keep it as extra profit is unknown, and somewhat irrelevent - you've already paid it. The revenue share part of the call price is listed here:
http://www.ofcom.org.uk/static/numbering/s8_code.txt and the revenue share depends on the number called. You need to know the next 6 digits after the 0.
2. the profit that your phone company adds to the call price; especially noting that if you call from a BT line this is zero pence per minute - BT is regulated to not add anything to the price. Other landline operators may add several pence per minute or more to the price. Mobile operators may add 20 to 40 p per minute above the regulated revenue share amount.
0871 numbers are regulated by PhonepayPlus with similar rules to those in force for Premium Rate Services (PRS) numbers beginning 090 and 091.
0871 numbers are different to 0870 numbers.
0870 numbers used to be "national rate" until 2005. Since 2005 they have just been "revenue share" numbers. Since 2009, revenue share was removed from 0870 and some landline operators offer 0870 numbers as inclusive calls.
0871 numbers are NEVER inclusive.
Rules for 0870 and 0845 numbers will be changing in the next few months to align with those for 0844 and 0871 numbers. 0870 and 0845 will no longer be inclusive. 0870 will regain revenue share status.
The Consumer Rights Directive will force "customer service" lines to move from 084 and 087 numbers to 034 and 037 numbers. Those types of calls will then be usable within inclusive minutes from both landlines and mobiles.
Where a company continues using 084 and 087 numbers, the new rules for "unbundled tariffs" will force them to declare the revenue share premium that is built into the call price.
See also:
http://en.wikipedia.org/wiki/Non-geographic_telephone_numbers_in_the_United_Kingdom#0871.2C_0872_and_0873_prefix