Reverse charge calls can be delivered by BT, or a service called 0800REVERSE. Charges for these services can cost over £5 for a call lasting just a minute. When you pick up the phone you get a message telling you who is calling, and giving you the option to accept the call and charges. The BT service uses a live operator, the 0800REVERSE service is automated.
These services are often used for making prank calls because the prankster knows that as well as disturbing you with a call, they could cost you a lot of money.
It is possible to block calls from 0800REVERSE (call them on 0845 0 779 000 or email firstname.lastname@example.org), but be aware that by blocking this service you may prevent friends and family getting through to you in an emergency - perhaps if they have run out of mobile phone credit.
trueCall can be used to block unwanted reverse charge calls.