Check out this article from NY Times. Free registration is required.
A couple of excerpts:
Its motto, �One number for life,� pretty much says it all. At GrandCentral.com, you choose a new, single, unified phone number (more on this in a moment). You hand it out to everyone you know, instructing them to delete all your old numbers from their Rolodexes.
From now on, whenever somebody dials your new uninumber, all of your phones ring simultaneously, like something out of �The Lawnmower Man.�
SWITCH LINES Anytime during a call, you can press the * key to make all of your phones ring again, so that you can pick up on a different phone in midconversation, unbeknownst to the person on the other end. For example, if you�re heading out the door, you can switch a landline call to your cellphone � or as you arrive home, a cell call to a landline, in order to save airtime minutes.