0800 are the free calls, 0870 and the like are the ones where they charge, or can charge extra to phone them.. 10p a min rather than say a standard 4p a min rate. It's explained better on that website. (click the red 'say no..' writing)
By searching for their normal landline number (like 01223 for cambridge) you find the normal rate lines.
I think Orange is 8p a min on their 0870 number, on the 'normal' regional number you just pay the standard rate for a BT call
Unfortunately that site only applies to UK at the moment.
