Found this website, I know it should be in the website links but it's probably best everyone reads it.
What happens is you enter the number of the company you want to call (Either it been a 0870 or 0845 number) it then searches for the landline number.
So if like me you get free calls to LANDLINE and MOBILES only, if you are near a computer, whack the premium/local rate number into the website and then it gives you the landline number.
For Example.
0845 611 5000 is Morrisons HQ.
Landline
01274 261000.
Try it.
http://www.saynoto0870.com/