The scam works thus:
A smartly dressed woman turns up on your doorstep and asks to use your phone because her car's broken down and she needs to call her husband.
If you let her use the phone she phones a premium rate number,(rumoured to cost £50 per minute) and pretends to be talking to her husband for a while.
After using your phone she will ask if you want money for use of the phone and offers you £1. The good natured will say 'No, keep your money' and when your phone bill arrives it'll have a call costing £250 on it!
It's not illegal and there is no criminal offense commited so watch out.
If anyone asks to use your phone you can decline politely by telling them that mobile and premium rate calls are barred from your phone and point them in the direction of the nearest public phone box.
Here's another unrelated phone tip:
If you want to get rid of telesales, use the magic phrase "I'm unemployed and have no money" - they hang up immediately.