Someone has probably already made this comment but has a newbie I would be unaware of it.
Anyway wouldn't it be great if we all changed our numbers to 0870 but kept our GN number top secret for relatives & friends only. Then when we fill in forms for credit cards, tax returns etc we insert our 0870 number in the box which asks for our telephone number. That way when our details are sold on to marketing agencies (even though we have ticked the box which forbids it!!!) and we get a call from anyone who is not a friend or relative, we can take comfort from the fact that we are getting paid to talk to someone, possibly in a call centre in India!!  You could even take advantage by asking them to hold whilst you go for a pee, put the kettle on, answer the door, ick the kids up from school or paint the house.

Look on the Useful Information & Say no to 0870! Hot Topics stickied at the top of the Chat section, near the top there is a list of threads discussing this.