I signed-up with WhistlePhone:
And I got a free phone number that gives me free 20min calls to any number in the US-- and it can receive calls as well!
There are other providers out there too. If you are in Canada you can use FreePhoneLine:
I'm sure there are equivalent services in other places in the world.
Anyway, suffice it to say, my tablet is now a phone and it makes it that much more awesome!
PS: you can also install LinPhone on your Linux box and there is probably a Windows equivalent...