T-Mobile has announced it is giving some of its smartphone customers unlimited free calling, when you are within range of a Wi-Fi signal.

This new feature is available on Even More and Even More Plus postpaid rate plans for customers with Wi-Fi Calling-capable phones. Wi-Fi Calling is based on the Smart Wi-Fi application that comes pre-loaded onto many of T-Mobile’s latest smartphones.  It comes from Kineto Wireless, which provides a similar app for Orange UK and Rogers Wireless customers in Canada.

When enabled, T-Mobile customers will see a blue ‘talk bubble’ icon in the status bar.  Once active and running, all voice calls made on your phone while within range of a connected Wi-Fi signal are reportedly not counted against your plan minutes.

Judging from anecdotal reports across the web, T-Mobile customers have been able to add the free calling feature to their accounts as of last Friday.  The fastest route to a quick activation is calling T-Mobile customer service.  Those subscribed to a Family Plan must activate the feature individually for each smartphone on the account.

Wi-Fi Calling is primarily pitched as providing a solid signal where none exists, a helpful feature for T-Mobile customers who find reception less than robust indoors.  Offloading wireless traffic to Wi-Fi benefits T-Mobile as well, reducing demand on its cell towers.

The technology differs from femtocells — small devices that connect with your broadband connection and deliver a 3G wireless signal in your home or office.  Because the Smart Wi-Fi app that powers Wi-Fi Calling is software-based, there is no hardware expense and little customer configuration required.  But Wi-Fi Calling is more restrictive.  A femtocell delivers a 3G signal to any nearby device registered to access it; Wi-Fi Calling only works with phones pre-equipped with the feature.

T-Mobile is also reportedly readying its own femtocell solution for low signal areas.  Their Cel-Fi Microcell is undergoing focus group testing at a price point of a $50 refundable deposit, and a monthly cost of $1.99.

T-Mobile’s website has created some confusion over their Wi-Fi Calling by delivering contradictory information to what customer service representatives are telling customers.  Customer service and an internal company memo suggest the use of the feature does not count against plan minutes, but their website says the opposite.

T-Mobile’s latest innovation begs the question: Would AT&T  — potential future owner of T-Mobile — ever offer Wi-Fi Calling to its customers for free, with no deduction of plan minutes when used?

AT&T femtocell users find the company does deduct plan minutes, unless customers pay for a $19.99/month add-on plan for an unlimited calling option.

[flv width=”640″ height=”380″]http://www.phillipdampier.com/video/Smart Wi-Fi.flv[/flv]

Kineto Wireless produced this video explaining how Smart Wi-Fi Calling works, and we’ve included a second video from the company explaining how to access the application from a T-Mobile smartphone.  (6 minutes)