It's an even narrower Verizon-specific thing, where you can add 1-5 extra "devices" to your monthly subscription (at $10 each) and get your calls, texts, etc on all of them. It's not a local pairing thing either, each device needs its own LTE connectivity, I guess you can pick whichever phone or smartwatch or whatever you like off your nightstand to match your outfit or car or mood. I don't use Verizon, and I've never known anyone who uses this feature.

(The whole concept reminds me of "Weird Al" Yankovic's "douchey" character from the "First World Problems" video, who pulls a second phone from his pocket to tap out the complaint: "some idiot just called me up on the phone, OMG, don't they know how to text?")