I’m sure there are other solutions but Google Voice might be just what you need…you would get a new number, which when called rings other numbers (Like your cell, office, home, etc). It also logs everything that comes in and out, and can receive text messages too! Voicemail is transcribed to email…some nice features! And if you wanted to, you can (eventually) port your number to them if that’s a big deal.
I would assume that any VOIP provider would charge you per minute for forwarded calls eventually as you are using there equiptment to route your calls so they will incure a charge.