Will tmobile let me add a different phone to our line?

Sadly, I just finished paying my iPhone 5s and it got water damage. I don't have insurance anymore. I doubt they ll be able to do anything with my damaged phone =l so I was wondering if I can buy an used iPhone from the same carrier and just replace it? Will everything be the same again?

As long as you pay the bill, you can use your SIM in any smartphone that's either on t-mobile or unlocked.

Though if your contract is up, see what offers they have on new phones; you may find you can get a brand new one from them for less than a secondhand one, as long as you agree to stay with t-mobile for another two years.