Sorry if this has been covered before, but just in case. If say, you need to call sky or a bank etc, which has a mumber that starts with an 0870 try the following website which will provide an alternative 0800 number (not always but worth a try). This will get you to exactly the same place as the 0870 no.
http://www.saynoto0870.com/
Click on "search to find an alternative number"
You'll get 2 options, search by company name or telephone number.
I tried the Sky 0870 2 40 40 40 (customer services) and it came up with a freephone number (0800).