T-Mobile GSM-over-IP Phone

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T-Mobile's new service allows you to use a home WiFi network or a T-Mobile hotspot to make VoIP calls. The services comes with a Wifi Access Point you can use at home.

There is no SIP or H.323 client on the phone; It creates a tunnel for GSM data on top of the IP connection established over the WiFi connection.

As for WiFi-GSM handoff:

These phones hand off your calls from Wi-Fi network to cell network seamlessly and automatically, without a single crackle or pop to punctuate the switch. As you walk out of a hot spot, fewer and fewer Wi-Fi signal bars appear on the screen, until — blink! — the T-Mobile network bars replace them. (The handoff as you move in the opposite direction, from the cell network into a hot spot, is also seamless, but takes slightly longer, about a minute.)

Will the call get dropped during handoff? Will the user notice the audio quality during handoff? The story does not say anything about it. But it seems for cell-to-Wifi handoff, the call will be dropped.

At the moment, you have a choice of only two phones: the Nokia 6086 and Samsung t409. Both of these are small basic flip phones (both $50 after rebate and with two-year commitment).

Someone on slashdot mentioned Nokia has similar phones in Europe:

Nokia launched the 6136 last Feb (2006) in Europe:
https://news.zdnet.co.uk/communications/0,100000008 5,39252128,00.htm [zdnet.co.uk]
This does the roaming wifi/GSM stuff as well.
Tested in Oulu, Finland in 2006:

https://www.mobiledia.com/news/49241.html [mobiledia.com]