T-Mobile Digits frees your phone number from your phone.

T-Mobile, man. That company (TMUS) is just determined to mess with the traditional cellphone-carrier model.

This is the company that eliminated the two-year contract. That lets you stream all the music and video you want, without counting it against your data allotment. That eliminated international roaming charges.

And now, the company wants to give you another phone number.

It’s called T-Mobile Digits, a new service, currently in free beta testing for T-Mobile customers. The central idea is that Digits breaks the one-to-one bond between your phone number and your phone.

In so doing, Digits introduces all kinds of new flexibility—and complexity—into your digital life. They fall into one of two categories, which I’ll call “One to many” or “Many to one.”

When you sign up for the program, you can choose one of those categories, or both of them. (Told you this was going to involve some complexity!)

One to many

When you sign up for this option, your one T-Mobile number rings all your phones—even phones from Verizon (VZ), Sprint (S), or AT&T (T), and even WiFi-only devices like tablets! The idea is that you can now take and make calls, and send or receive messages, from whichever phone you have with you.

(If you’re hoping to include a non-smartphone in your arsenal, T-Mobile will send you an SIM card for it. Which means you’ll have to replace the phone’s existing SIM card. Which means it will have only one phone number. Which means you won’t be able to use the “many to one” feature described below. Did I mention that this all gets a little complex?)

When someone dials your one T-Mobile number, all of your associated gadgets ring at once. When you pick up one to answer it (or decline it), all the other devices stop ringing.

So why is this useful? Well, if you race out of the house and forget your phone, no problem! You can borrow a phone, or you can use the Digits website. You can still get your calls and texts.

Or maybe you want to take a cheap ratty old feature phone with you running or rowing, instead of carrying your precious $700 smartphone. No sweat; now that ratty old feature phone can get (and send) your calls and texts.

Heck, you could now treat cellphones like multiple extensions of a landline. You can keep one phone up by your bed, and another downstairs; pick up whichever one is handiest when someone calls or texts.

What’s cool is that you can access your calls, texts, and voicemail from any device. From a watch. From a WiFi tablet. From any computer, tablet, or phone in the world, via the Digits website.

David Pogue.

Full story at Yahoo News.