Telephone numbers beginning with 0871 are business rate numbers. To call a number beginning 0871 using a Three Mobile line will cost you the same rate whether you have a pay as you go plan or a pay monthly contract.
Phone companies charge an access charge to phone business rate numbers such as 0871. The company itself will also charge a service charge to ring their phone line, this is regulated by the Phone-paid Services Authority and can be upto 13p a minute. This is the same whether you call from a mobile or landline.
The access charge to call an 0871 number using a Three mobile is 45p a minute plus the service charge (up to 13p). The total cost of the call is the access charge plus the service charge. Therefore the maximum charge you can expect to pay for ringing an 0871 number is 58p per minute.