LG Watch Sport NumberSync issues with HTC 10

[NesToPROD]

Hi folks, my apologies in advance if this is a less than noble question.

I have an unlocked-straight-from-HTC-on-launch-day HTC10. Over the weekend I bought the LG Watch Sport from AT&T because google was sold out and I'm impatient. I like the thing so far, but I can't for the life of me get the thing to give me a sync code for NumberSync, it just tells me to try again later. I took it back into AT&T and the guy there looked up whatever he looked up and, to my cheers of joy, showed me that I could REPLY to his text with the watch and my number would show up. Me being a first time smartwatch guy, I figured that was confirmation enough. Now I've sent texts with and no-one recognizes my number, so obviously the REPLY just went through the phone and that guy let me think it was fixed just to get rid of me.

I've read a couple posts on different sites saying that it has to be an AT&T phone to work, is this true? Is my beloved HTC 10 that AT&T completely neglected? Is there a chance that it's just because the watch is so new and needs an update?