It's well publicized on various Reddit and SD threads that with Metro-by-T-Mobile, a lot of users pay only for the first month of new service with free (or cheap phone). Immediately after phone/service purchase, the phone gets put in a drawer for 6 months. Then, even if you don't pay for service for months 2-6, when you take the phone out of the drawer on day 181, you can unlock the phone using the unlock app and use the phone on another carrier (or flip it).
I just tried that with a Moto G Stylus that I bought on Aug. 11. I managed to unlock it this morning My cost was $25 for the service, with no activation fee. It's not clear whether that plan is still offered. I also had to port in an inactive Visible line and trade in good working condition iPhone 3G valued at $0.
The one big drawback is that Metro resellers are reputedly very aggressive and deceitful in pushing extra charges, such as insurance plans, activation fees, and state/local taxes. You can usually, but not always, avoid those charges by going to a corporate store, which are few and far between and usually located only in cities. I got my G Stylus at a corporate store within a mile of my house, although it's in a shopping mall that's across from a housing project.