I found this great little thread:
http://forum.o2.co.uk/viewtopic.php?...8db2d8df25b145
If you're with o2, a vast majority of 0800 numbers are not free and cost £00.20p a minute. (I'm not sure with other networks)
if you dial this number:
0200 222 0700 and then when asked, key in the free phone number and wait, and wait, and wait (Around 20 seconds of silence) you will be connected free of charge.
I just done it to renew my insurance. It takes the minutes from your contract so it wont charge you.