When my sister upgraded to the iPhone 6, she was almost done with the 2-year contract she had on her iPhone 5 with AT&T. She called and asked how much longer she had to wait to buy the iPhone 6 and get a new contract. AT&T, of course, told her she didn't have to wait and that she could switch now and be on one of those plans where you get your iPhone "free" as part of your plan. You can upgrade whenever you want!!
Now, getting an iPhone 6 isn't mustachian... but she knew enough to know this was really dumb. Paying for the phone and then paying separately for service is always going to be cheaper, even if it means you have to keep your phone for at least 2 years without upgrading it.