Finally worked after 3 visits!!!
1st Visit (Friday):
Spent 2.5 hours at the store trying to get approved, but the apple employees didn't really know how to do the upgrade program through T-Mobile, and then got declined because of Citizens system not working. Finally told them I just wanted to buy the phone, but they somehow ended up financing it through T-Mobile...
2nd Visit (Saturday):
Was at the store for 1.5 hours and iUP was working, but there was a glitch with T-Mobile so they could not get iUP to work for T-Mobile customers. Was about ready to give up at this point.
3rd Visit (Sunday):
Went back today and spent 2 hours at the store. First there was an issue with returning the phone (got an error saying returning the phone would not cancel the T-Mobile finance agreement, so had to call T-Mobile and sort that out). After that was all taken care of and they could process the return, signing up for iUP and getting approved took 10 min.
Happy finally worked out and I didn't give up. For reference, my credit sore is 775, and used my Discover card to sign up with a limit of $5600 and cash back limit of $400.
As a fellow T-Mobile customer, I also had to make multiple visits to the Apple store trying to sign up for iUP--except I'm still unable to join. After logging into the tap.t-mobile.com website to "upgrade" my line to my new iPhone 6S Plus, none of the phone lines from my family plan were listed on the website as eligible.
Upon calling T-Mobile to report this issue, there was mass confusion about the word "upgrade" because T-Mobile assumes they're referring to JUMP! (I had JUMP v1, but got rid of it in anticipation of joining iUP). Several phone calls later, Apple simply points the finger at T-Mobile saying that I have a "weird" account setup and T-Mobile points the finger right back at Apple saying that their system must be acting up.
From what I understood, the Apple store needs to upgrade my phone line to the new iPhone (via their T-Mobile portal) as part of the process to retrieve a QR code which will then be used to associate the phone to my account on the Apple store's end. I couldn't even get to the point where I would apply for the program. I ended up buying the phone at full price.
Can anyone who went through the iUP with T-Mobile comment on how the upgrade and activation process went? Better yet, did anyone have the same issues that I had? I couldn't seem to find anything via search.