this is from t-mobile rep
we are looking at the activation of the watch's SIM.
I did some digging and I have found that the issue actually lies on our end here.
At this time business accounts that purchased an Apple Watch Series 3 from a retailer other than us aren't able to activate the line needed for the watch due to the way the new lines are activated with the On Device Activation that the watch would normally use. I know this is frustrating, to say the least... The good news is that it looks like at this time our support teams are actively engaged in overcoming this as quickly as possible, as we understand how imperative it is that we have this up and running asap to our valued fam!
As they are currently working on a solution, there is no estimated time at this time. I know this is far from what you were hoping, and I truly wish that there was a way to keep you in the progress loop, without needing to make frequent side trips. I myself prefer having all details in order so to know what to expect! While we are a bit tied up there, there are a couple things you can do gain use of your Apple Watch:
1) you can keep the watch you currently have, and pair it via Bluetooth until the network activation are resolved
2) if you have some time to spare, you can return the watch, and reorder through T-Mobile. However, at this time the watches have been a highly popular item and is back ordered at this time.