BT started charging its customer 6p for every time you press 3 to return a call, so if its a mobile and its switched off you will still pay 6p! So using the service 10 times in a month would cost 60p.
And don't forget, all service providers except a few CPS providers have a minimum call charge which relatively the same thing. So if you are on BT Together option 1 and make a daytime call lasting 1 minute, you will be charge 5p rather than the advertised 3p per minute charging. NTL charges the same known as call connection charge and if call lasts just under a minute, you will be charged 5p too.
Another thing, BT didn't even inform customers about this charging on 1471. Although there is a message 'There is normally a charge for the service'. People assume that this means calls will be charged at the standard rate.
Also, BT's billing system was rated the worst, people found that bills from BT were confusing and contained a lot of hidden charges. When I was last with BT, the bill showed 330 calls to a number in Guernsey starting 01481 and was the same number all the way down the page, I called the number and the number wasn't even in service and contacted Guernsey Telecoms now known as Cable and Wireless guernsey and they said the number has never even been in service before. BT credited this back until it happened again and this time it was a 0845 BT dial up internet access number with calls lasting 2 hours or so and was listed all the way down the page of the bill with total cost £110, It took over two months to rectify this and nearly had to pay it as Oftel was a complete waste of time!