I'm using Tello as my service provider. Until now it was running on Sprint towers but at the end of October they are ending with Sprint and transferring completely over to T-Mobile. I don't know anything about T-Mobile and I wanted to know what are the best flip phone options without any compromised service. Does anybody have any info for me? Please respond asap. Thank you very much!

I would recommend you to take the shock phone

YEG Same here. I'll probably be switching to US Mobile and get a Verizon phone.
(https://www.usmobile.com - "US Mobile offers 5G and 4G LTE coverage on the 2 largest cellular networks in the US with custom and unlimited prepaid talk, text & data plans.")

@YEG I too am on tello, wanted to use my Sprint sonim phone, then found out they weren't using sprint anymore... instead im using the schok phone. Its decent but i wouldn't mind a better less glitchy more user friendly phone. The sonim flip att version is probably a good choice, just confirm it works with T-Mobile towers, im pretty sure it would.

There is a website frequencycheck.com that will show you every phone what bands it uses and which carriers it is compatible with

esaphire3
great website - doesn't seem to have the classic flip L125DL on it