Numbers starting with the 0871 prefix are business rate numbers and whilst you wouldn’t be charged for receiving a call from an 0871 number, you are charged for making a call to one from your landline.
Currently, from a landline you will be charged up to 13p per minute (this charge is decided by the company you are calling and they will tell you how much this is) plus an access charge from your own phone company (this is the usual per minute charge they apply). In other words, there are two parts to the cost of your call to an 0871 number.
In the UK there are no phone providers offering 0871 numbers within their “free minutes”, so remember, even if your package includes free minutes, you will still be charged for calling an 0871 number.
The reason an 0871 costs more than, say, a local call is because they include a service charge, to cover whatever service you are calling up to use. For example 0871 numbers are sometimes bought by ticket-selling services and for a company’s customer service line.
0871 and other business rate numbers are regulated by the Phone-paid Services Authority (PPS).