Yesterday, an acquaintance of mine who owns a business asked my how I would solve the problem of making it easy to verify someone's ID in the absence of a wallet or other credentials, without using biological markers (fingerprints, retinal scan). In other words, easy and without the use of special equipment.
My first thought was to use a cell phone. There's no reason why I couldn't text your number to a special number and have us both receive the same 4-digits number to confirm with each other, along with my name.
It could also be used to get into locked buildings. The phone has a GPS device already, so a simple app could message my location to the number and pulse the lock mechanism for those with electronic access.
No idea if he likes the idea or not, but I forwarded it on.
I looked up a few domain names, which is what I usually do for things like this, and I discovered that idbyphone.com is already taken. By Microsoft. The domain doesn't go anywhere, but they've owned it since 2007.