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What's a T-Mobile Digits?

T-Mobile last week launched a program, called Digits, that changes the way phones register to its cellular network. To the casual observer, Digits looks like just another multi-number VoIP application. Since this is already a popular feature of UC applications and consumer apps such as Viber and Skype, it is easy to dismiss. While Digits does share some characteristics of these applications, it is actually much more.

Part of the confusion around Digits is it is less an application and more of a change in architecture. T-Mobile confuses the issue by explaining Digits with "new" yet familiar features. What's happening is Digits enables a new approach to authentication that creates a new set of capabilities. T-Mobile will grow these over time, but the short-term examples already exist, albeit in a different way.

What Once Was Doesn't Need to Be

Digits decouples the telephone number from the cellular SIM -- a significant breakthrough. Currently, or prior to Digits, cellular carriers pair a telephone number with a SIM card in a one-to-one semi-rigid relationship. When a phone is activated it gets authenticated by the carrier's Home Location Register (HLR) that validates the SIM and telephone number.

This architecture is why cell phones only support one native number and why that number is tied to a single device. It falls into the "way it's always been" category, so we generally consider it normal. However, other forms of smartphone communications, such as email and social apps, work across multiple devices.

Cellphone calls and messages have always been associated with a single device (or SIM card). We've overcome this restriction with various tricks, such as over-the-top apps that use the data channel and call forwarding. But Digits offers a network-based approach.

A Digits telephone number has a dynamic relationship with the SIM card, and gives the control to the subscriber. A user can have a single, native number shared across multiple (up to five) devices, and/or a single SIM card that supports multiple telephone numbers.

What makes this different than popular VoIP applications is that Digits utilizes the cellular voice channel instead of the data channel. That's a distinction that confuses many, but the data portion of the cellular connection is less robust than the voice channel. T-Mobile calls the voice side "carrier grade" because it and other cellular carriers prioritize circuit-switched, voice traffic. Overall the benefits of the voice side include higher quality of service, 911 support, broader support over older and slower networks, and preconfigured domestic and international roaming.

Lost Phone? No Biggie

Digits routes calls to a user, not a device. This is a familiar evolution we have seen with VoIP, email, and other forms of IP-based communications. It's why you can log into email or Facebook from any device. There are quite a few implications associated with the change.

Digits works best with Android-based devices on the T-Mobile network, but it is not restricted to either. Each device will sync the phone number, as well as call history, voicemail, text messages, and contacts.

Users have quite a bit of flexibility on devices. They can select from several Samsung phones preconfigured for Digits, or download the Digits app to smartphones of their choice. In theory, any Android smartphone should offer a similar experience via the installed app. iPhone support comes with a few caveats. iMessage synchronization needs to be turned off since it conflicts with Digits synchronization. Also, the iPhone's experience is less seamless due to restrictions Apple imposes on system-level alterations.

When using the service on a T-Mobile connected phone, users get an end-to-end T-Mobile carrier-grade experience. When accessing the service from a device using a different cellular provider, the service still uses the voice network, but goes through a T-Mobile point of presence. A user can also access Digits services from a computer over an Internet connection, but not from a wired phone at this time. If you forget or lose your phone, you beg, borrow, or steal another and make it yours in seconds -- natively.

Digits could usher in a new chapter in BYOD. An employer could simply provide a Digits account for use on an employee's personal smartphone (any carrier) or desktop. A single cellphone can use Digits to support a secondary, native number. Not only will usage charges (such as international calls) be automatically separated (and logged), but so will messages and contacts. All of the history and contacts will disappear when the Digits account is removed.

Since a specific phone itself is no longer critical to communications, phones could become more like jewelry -- a smaller phone for an evening out, a waterproof phone for the beach, and the ability to simply swap phones mid-day as a solution to battery management, for example. Email and other apps are already synchronized, and now with Digits so are calls, call history, and messages.

Both Digits and Verizon's One Talk services allow a single number to work natively across multiple devices. However, there are several key differences between the two services. One Talk supports video and wired desktop phones. Digits supports multiple numbers on a single device and includes a user portal to manage devices.

T-Mobile launched Digits under a beta program (possibly a first for the cellular industry), and plans to make it generally available early next year. It is a foundational change to infrastructure, so new types of services will be added. For example, it could extend to enterprise wired phones via an IMS PBX such as Mitel's MiCloud Telepo.

For now, T-Mobile is confident Digits is an exclusive feature. T-Mobile claims to have developed this over several years and credits Samsung, Ericsson, and Mitel as key contributors. Mitel's Mobility division supplied enhancements to the Telephony Application Service and IMS core, and both companies claim new patents that touch just about every part of the T-Mobile network.

Dave Michels is a contributing editor and analyst at TalkingPointz.

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