Does your internet access include telephone calls, or do you have a pre-paid bundle for landline phone calls?
If you do you probably still have to pay for 0870 and similar numbers. However help is at hand at a web site that you can search for alternative landline numbers to those advertised by companies, government and so on.
Just go to http://saynoto0870.com and search for the organisation you want to call. You can also add landline numbers yourself to the web site. We used the service to get a no extra charge call to National Express East Coast customer services.