What if you had a wristband that acted as a SIM card?

That possibility is being raised following word that Japanese giant telco NTT Docomo has developed a prototype Portable SIM unit, which will be shown tomorrow at the Mobile Asia Expo 2014 in Shanghai.

Currently the size of a credit card but intended for smaller wearable form factors, it also contains Bluetooth and Near Field Communication (NFC) and, the company said, could potentially harbor other capabilities.

When waved near a phone, tablet, videogame, or other device, the Portable SIM could transfer a user’s caller info and identity to that device for families or businesses that wanted to share hardware. Gone would be the days when you needed to insert a SIM card into a phone for it to be yours.

“Here’s the problem,” Endpoint Technologies Associates analyst Roger Kay told VentureBeat. “The phone already has that position [as an identifying device]; it’s portable,” and, he added, in some sense it is a kind of wearable.

“You won’t be going out without your phone,” he pointed out, raising the question of why you might want a separate identifier.

But Docomo is suggesting a future where you might go out without your phone. A spokesperson told Tech In Asia that Portable SIM could allow a user to leave home without a smartphone and, when you wanted to check email, you might waive your Portable SIM wearble in “front of a vending machine, perhaps” to display your info.

You could also have two separate identities – say, work and personal — for the same phone, using two SIM wearables. But there are already containerized solutions for work-personal separation that don’t entail having to deal with a phone and two identifier wearables. If lost or stolen, Docomo says the Portable SIM owner could implement a kill command, just like a phone.

A Portable SIM card embedded in a wearable could also be used to log on to sites or unlock doors. But it’s then another key to carry, which has to be disabled remotely if lost.

The success of a Portable SIM approach comes down to whether users want to physically separate their phone identity from their phone.