T-Mobile released support for WiFi calling and hotspot a week or so ago.  I've been waiting for this for quite some time since the two biggest problems I've had in recent months have been cell signal coverage, and being able to use a hotspot when I'm out and about.  

I've had the update for a bit over a week, and it's working very well on both counts.

WiFi calling has been tried before using UMA, mostly for BlackBerry phones.  T-Mobile had the Hotspot @ Home service that allowed you to use your internet connection to make phone calls from your cell phone.

To setup WiFi calling is very simple.  A new app shows up called WiFi Calling, which allows you to turn the feature on.  Once it's on, and if an appropriate wifi network is available, it logs into T-Mobile's site remotely and you're done.  

I've noticed a few things when using WiFi calling: 

Normal phone network connection is disabled – this is actually a good thing for me, since several places I frequent have awesome wifi coverage, but have horrible T-Mobile coverage.  Since the normal phone radio is off, it saves a huge amount of battery.  However, when you leave a wifi coverage area, it takes a few seconds for the phone to spin up the normal radio and get back on T-Mobile's normal network.  Can be annoying too if you're in the middle of a call – oops!

Jumping wifi AP's works fine - I've been at work and jumped across a number of similar AP's, as well as onto a completely different AP, and things work great.  You can't continue a call during these times, but that's OK.  You just have to be a bit more cognizant of when you're going to go out of range of an AP as you will drop your call.  I've also noticed going from home to work to friends house doesn't require any intervention.  My G2 was joined to all of these three networks, and as soon as I'm in range, a few seconds later I'm using wifi calling.

Battery life increases – as I mentioned above, because the normal radio is off, I'm seeing a 20-50% savings in battery depending on where I happen to be during the day.  Many times I would come home at night and be near 20% battery left.  Now, I come home with 40-50% left!  Quite a big difference.  Remember, though, that my work location has no T-Mobile coverage, and so I'm on wifi for 8+ hours.

Call quality and speed excellent – people can call me and my phone rings almost immediately, instead of the 1 or 2 rings in the past that were used while the towers were searching me out.  In addition, the speed of text messages and outbound calls connecting is much faster.

I've connected my phone to a wifi system where I can take packet captures, and from what I can see, the wifi app establishes an ipsec connection to T-Mobile through the internet, and periodically updates it's status through that same connection.  I plan on doing some bandwidth profiling and other information later this coming week and will post that information here too – I'm particularly interested in how much bandwidth it takes for a phone call as well as for text messages.

The hotspot functionality is similar to the same options available on the Droid series of phones from Verizon, and the EVO on Sprint.  However, the biggest difference is the network.  Here in Seattle near my house I get almost 10Mbps download, and 4Mbps upload, with ~40ms of ping times – that's pretty fantastic from a mobile connection, and far better than the Clearwire/Sprint 4G stuff I've used before.

I hope that more phones adopt the wifi calling like feature, including from other providers.  This will allow us to use more minutes and be more connected in many places that are completely dead now – basements, work, etc.  Unfortunately, a lot of providers are resisting the shift to internet technologies for voice – even Verizon is saying their new LTE network will not support data and voice via the same technologies.  

For me, it makes the G2 and T-Mobile's network more reliable than any other provider in all of the places I frequent.  Great feature!