I have this with Sky Talk, sounds like the same thing. It's great. If they had cable internet here instead of broadband, though, I'd use that and not get BT phone lines. Then I would get Vonage, which for a small fee more gives you a U.S. number for relatives to call you on in addition to a UK number. The whole package price would end up the same or less than the BT phone line.
Anyhow, it seems like the times I've gone over an hour and didn't hang up and call back but rather kept on talking, the per minute rate they charge is still really low (this is with Sky Talk, don't know about BT).