So Here is the question/story,

Ive got a ZTE Obsidian phone thru Wal-Marts Family Mobile service, from what i can tell they operate through T-Mobile. (everytime ive got reboot my phone and such the T-Mobile logo pops up someplace...hence why i believe its a T-Mobile based phone) I keep seeing these phones on Amazon and Ebay (a Galaxy S5 or S6) that you can get as unlocked or T-Mobile and ive been considering getting one.

The Question is if i get the T-Mobile one could i just plop my current sim in the thing and have it work?, or would i be better off going with the straight unlocked version?

Also if i do this would i have to call in with the IMEI number and all that crap to T-Mobile/Family Plan as well?

Im a relatively tech savy person, but this would be the frist time i have EVER done this with a phone so plz explain the process to me as if i was a Nooblit.

Thanks and have Good One