Ha, this is the old, old TMO @Home service (2nd gen was Voip from a router). It is also called UMA (unlicensed mobile access), but is really only available for Blackberry devices. Basically it was TMO attempt to use femtocells.
So it was free to use (all other carriers charge you for hardware and a monthly fee) but you used your minutes, or they had a $10 add on that gave you free calls over wifi.
UMA on Android is a top issue (#26 by number of stars)
Issue 6242 - android - UMA enable Android OS - Project Hosting on Google Code
So vote for it!
But there are reports that this is coming to TMO Android devices later this year.