I was one of the unlucky few who had T-Mobile issues. It took 3 hours and 8 different T-mobile employees to get it taken care of and I'd like to share what I learned so I can hopefully help anyone in my situation fix it sooner. Please up vote this so other T-Mobile customers see it.
I have a business account and during the cellular activation it would tell me the last 4 of my social I was inputting was incorrect. Here are all the different things the first 7 support employees told me, none of which are true.
1st employee: You just need to enter the last 4 of your Tax ID since it's a business (didn't work, either)
2nd: T-Mobile Apple Watches are unable to activate unless you buy it from T-Mobile, you'll need to return your watch and buy a new one from us (obviously not true since it's now active)
3rd: You'll need to activate it on a temporary personal line first and then transfer it to your business line, let me send you to our activation department (nobody in activations had any idea what he was talking about)
4th (in activations): You need to take out the sim card and read the ICCID number to me (Apple Watch doesn't have a sim card, the sim is built in. He was looking for the EID number which is on the box, even though when I tried to give him that he said it wasn't the same thing. It is)
5th (activations dept manager): Apple forgot to send you a sim card with it. You'll need to buy one from T-mobile (again, obviously not true)
6th (in business customer support): If it doesn't take your SSN on the activation screen then Apple sent you a faulty device and you'll need to get another one (definitely not true, it's active now)
7th (T-Mobile Apple Rep): Said the EID/ICCID needed to start with 8901 and since mine started with 8904 it was actually an AT&T model and Apple had shipped me the wrong one (again, not true since it is clearly active on my T-Mobile network now).
Solution: I finally went into a T-Mobile store and the guy there read to me an internal email saying that at the moment, business accounts are only able to be activated on the Apple Watch if it is bought directly from T-Mobile and that they are working on a fix but there was no estimated time for this to be finished (the second guy was close). I had three options, return my watch and order one from T-Mobile (wouldn't get it until October 30th), only use it on bluetooth until the fix was implemented, or switch my account from business to personal. I went with the last one and I'll switch it back whenever T-Mobile finally get's their stuff in order.
I was an Apple employee for 3 years so it was pretty obvious at the time that none of these are true, though I know it's not obvious to everyone. I hope this helps. Again, please upvote this so people can see it. I created this MacRumors account just to post this.