I also tried that. I got a phone off contract for really cheap (I think it was Verizon) and then tried to port it to T-Mobile. My connection was really flaky. I finally filed a claim on my insurance for my phone and switched to a nexus 5x. That solved the majority of my problems. Also each network is divided between bands, and phones sometimes only focus on specific bands. It doesn't really make a difference if your phone was made for a specific carrier, but if you are trying to transfer networks your radio might not be able to fully take advantage of the new network.
