It's also worth noting some of the other changes:
http://www.callsave.bt.com/pricing.html
The minimum charge is being replaced replaced by a connection charge (a la ntl), however bt's will be 3p
Mobile calls are streamlined to 15ppm day, 8p evening and 5p weekend, which is cheaper for some, more expensive for others.
And the other significant difference is whole minute pricing, so a 1.01 minute call will be rounded up to 2 minutes, which takes me back to the old days of anologue mobiles!