If you have a telephone package with inclusive calls, chances are that calls to 0870 numbers are not included and will be charged at normal rates. Companies pretend that they are offering us a "national" service but really they are making money from us all. Companies get revenue from even national rate numbers. For a call centre this could make plenty of money.
Monst the 0870 numbers simply route through to a normal telephone line. Here's the "Say No to 0870" website which will help you find the "normal" telephone number for most companies, stores and helplines. http://www.saynoto0870.com/