That is exactly what I do, twice a year
Yes that means I can get two iPhones (or whatever I please) per year, at full subsidized price.
I have 5 lines with ATT, on the old family share plan. Two iPhones with unlimited data, and 3 feature phones (just going along with what ATT calls it
). My line has a contract end date the same year as one of the dumb phones, and the remaining two dumb phones the next year. When the dumb phone contract is up, I go to Best Buy and just tell them to do a cross upgrade onto my line. They know exactly what to do. Each of the dumb phone is $10/mo. You do the math. Don't assume people who sign 2yr contracts are suckers.
Yes I actually called ATT about this when I first thought of this "loophole", doing cross upgrade from a dumb phone without a data plan. They knew exactly what I was asking and told me I can absolutely do that.
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Data speed with T-Mo around my house absolutely kicks ATT's butt. I tried my friend's T-Mo sim in my unlocked Sony phone. It was ten times faster than ATT, both with LTE service here. If I didn't have it so good with ATT's subsidy, I would switch to T-Mo in a heartbeat for their awesome $100 for 4 lines deal.