In case you haven't noticed, T-Mobile really likes to antagonize AT&T. Only hours after AT&T announced its upcoming NumberSync service, T-Mobile let everyone know that it too is working on such a system. Apparently T-Mobile's will be better, not that either of them exist yet for us to verify that.
With AT&T, NumberSync will let you share the same account information across multiple SIM cards. So you might have an LTE-equipped watch that run on the same plan as your phone. T-Mobile's system will presumably offer similar functionality, but Tmo doesn't have shared data plans currently. So, there's potentially a bigger benefit to sharing a single number than there is on AT&T, which already uses shared data.
T-Mobile's Mike Sievert came out swinging saying, "What we are working on will make this version one offering from AT&T look small. Our strategy is to ask customers what they want and need, and then build it. AT&T’s is to build or buy things, and then try to convince customers they asked for it."
Sick burn. dude. That's all the information we've got about the service... so basically none. It's happening and T-Mobile says it's going to be way better than NumberSync. We'll see about that once they're both available.