Still thank you so much for your reply.
Sorry about my vague description. I am using Lycamobile's service now. The reason that I cannot buy a T-mobile based iPone 7 is I have to go to other countries for two months each year and I do use the new phone in the States.
Buying the new phone for one-time payment it's totally OK for me. Since there are other options and services existed, I want to know if I have the chance to buy the different iPhone program or not. But sadly I found that I could not buy the yearly updated program service so far because I do not have any of the four big carrier's phone number. Probably the only method for me is to move to one of the four carrier's service if I still want to buy the updated program service.
I just want the small group of customers who are not using the four carrier's service could also have a chance to enjoy the attracting updated iPhone program. This is why I said 'unfair' above. But who knows, maybe just only I have this problem, or I am too foolish to find the way to do that successfully right now.
Thanks @sberman.