Finally, a carrier decides to use a better technology for their customers.

Details are starting to emerge on T-Mobile’s new VoIP service that lets customers switch over to Wi-Fi when the T-Mobile signal is no longer strong enough to take a call.  T-Mobile explains it like this:

Wi-Fi Calling for Android is built upon the same technology as UMA, but is a different implementation from our past offerings. While T-Mobile’s current Wi-Fi Calling solution which operates on BlackBerry smartphones, for instance, seamlessly hands off calls from T-Mobile’s network to Wi-Fi networks, Wi-Fi Calling for Android does not.

A pre-installed application from T-Mobile will allow customers to make and receive voice calls and SMS over an accessible Wi-Fi connection.  This provides customers with better indoor coverage and in some cases may provide the ability to make voice calls in locations where they weren’t able to do so previously.

For consumers, Wi-Fi Calling for Android increases coverage and uses voice minutes. [NOTE: calls originate on Wi-Fi, but are carried across T-Mobile’s network once beyond the initial Wi-Fi connection.]  T-Mobile also has plans for business which can help defer wireless minutes and provide cost savings.

Wi-Fi Calling will be available on the new T-Mobile myTouch, Motorola DEFY and others Android phones in the coming weeks. We are planning to expand Wi-Fi Calling for Android to as broad a selection of Android smartphones in our line-up as possible.  Éclair (2.0/2.1)  and Froyo (2.2) are the Android OS versions that currently support the application.

Competing carriers like AT&T T , Verizon VZ and Sprint S use something called a Femto or Micro Cell.  That’s a product that usually costs $100 or so (though carriers are known to give them away free to louder customers).  These devices create “mini cell towers” in your home to reach the areas where the carrier’s real towers can’t.  This is a proprietary technology and isn’t compatible with other devices.

But most phones these days also have Wi-Fi …

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On T-Mobile’s Wi-Fi-Switchover UMA technology and on these Femto Cells, you get billed for minutes even though you are using your own broadband to make the call.  They’d argue that once the call leaves your ISP, the call is made over their lines.

I’ve had an AT&T Femto Cell in my house for month and it works well.  It does what it is supposed to do.

But where T-Mobile beats Sprint and AT&T is that you not only get reception in your house but you can get reception in your neighbor’s house.  And at work.  And at your parents house.  And the pub.

Anywhere there is Wi-Fi, you can take and receive calls on your phone number.  You can also pick up voicemail.  In an interesting customer-screw move, most carriers won’t let visual voicemail come to your phone over Wi-Fi (though the visual voicemail is just data packets like a web page or e-mail).

If you are out of range, you are out of luck.

An interesting side note which I haven’t confirmed (update: Confirmed) is that T-Mobile’s VoIP  may be a boon when traveling abroad.  Instead of jumping on a foreign call network at astronomical rates, you can just pick up and make your calls on Wi-Fi at your hotel or in a cafe.  This will save all kinds of money.

I should note, like T-Mobile did above, that the Android version of the service is inferior to the Blackberry version.  On the Blackberry, the service is transparent and switches automatically, in call.  On the Android, if you lose service during a call, you have to open an app on the phone and make the call again.  Femto Cells also offer this auto-switching advantage over T-Mobile’s Android solution.

But, overall, VoIP beats Femto Cells hands down.

If carriers don’t start adopting these types of measures to keep their customers happy, more and more will start porting their numbers to services like Google Voice which are giving customers these services and more for free.

T-Mobile should be applauded for giving their customers a better alternative.