I used to use a BT Chargecard, but often found it was little use in rural areas. Phones marked on my maps had either been disconnected or completely removed. The last one I tried to use was a few years ago on one of our most popular National Trails. I opened the door and could literally hear masses of spiders webs tearing in the process. It was like something from a horror movie, and it was clear that no-one had opened the kiosk door for a very long time. I wasn't surprised to find that the phone was dead.
I should also point out that some time ago I made use of a 'free' number to call someone. It was all set up on their account, so that I could call free, and they would be billed for the call. I was surprised to find, after using it on a hotel phone one evening, to be presented with a bill for £2 for a connection charge. Seems that the hotel didn't want anyone calling for 'free' if they could screw a couple of quid out of them! Just be warned that you might get charged the same way no matter what set-up you use!
Edited: 12/04/2012 at 14:27