If you are a registered Whistle user like myself, thn you probably already received this email that they are limiting the calls to 20 minutes in length for the free users. After that, there is a small per minute fee. This does not affect me at all, as I do not use it for that long, at least right now anyway.
If you are not a Whistle user, I suggest trying this app out. I'm not a hardcore user of it, but I do take advatage of free calls, especially for long distance in the US.