AT&T is almost ready to launch a new feature that ties together multiple devices under one phone number. The feature is currently called NumberSync, and the carrier expects it to launch on a single device (weird) very soon. More devices will launch later this year, making the feature actually useful.
The idea is that people are hesitant to buy multiple connected devices because they don't want to deal with multiple numbers and separate charges. NumberSync would allow customers to have the SIM cards in their watch, phone(s), and tablet all sharing the same line. If you get a call or a text, it should appear on all your devices no matter where they are. So it's sort of like Google Voice, except someone cares enough to support it. This would make a smart watch with LTE a slightly more logical option, for example.
NumberSync is a network-based technology, so OEMs will have to work with the carrier to support it. That's something AT&T hopes will become the norm. However, don't expect unlocked phones to work with NumberSync any time soon. AT&T doesn't expect to charge a separate fee for NumberSync, but you'll probably still pay the standard device fee to connect them to your data plan.