Can I still pay monthly for my device if I switch carriers?

Just switched to T Mobile—live with my grandparents so I’m the techie of the household, however as the account holders and payers, they get so panicked and start freaking out so easily and gosh it’s so annoying.

I’m in a bit of a pickle that may make them absolutely explode lol. We swapped to T mobile and it works incredibly well with 5G cell service (calling) and data. In town, that is. We live literally a mile down a straight road from the town, and the signal instantly drops to 1-2 bars at home here in this neighborhood. They both have “free phones” (iPhone 17e’s) and I have an iPhone 17 which I pay $35 a month for. They both pay for their lines and mine is “free”.

My conundrum is that if we switch back to our previous carrier, (Xfinity) who was amazing compared to what we get currently, what happens to the phones? My grandparents don’t mind handing their new devices back into T Mobile and neither do I. But can we? I called our local T Mobile store and they said if we scrap this plan we would then owe the entire balance of the phones.

If we truly can’t return them without owing anything, can we pay for the phones monthly still then instead of a lump-sum? Because they won’t mind doing that.