How do you bring a law suit against T mobil

I sent a unused S23Ultra back to Tmobil and I have my paperwork from their local store which is where I went they recieved the tracking which tells them the phone was recieved they caused me so much trouble with this issue its been 6 months of threatening to turn my phone off I have 4 phone lines people in different states that need there service they are insisting I pay for this phone I been there 8 yrs can't ever remember missing a phone bill to the poin Im in columbia with no service do uou know how hard this has become I paid for the international servive and it never worked the entire time Im here I know my family is going nuts.