WORST SERVICES EVER

I had 2 contracts with cell which ended in april 2021 and i contacted them for a paid up letter which i recieved on the 19th july 2021 and promised the account was closed and credit breau was updated ,however since august 2021 ive been recieving harassing EMAILS,SMES AND PHONE CALL stating that i still owe and legal action would be taken against me ,every agent i speak to ask for the paid up letter which i sent ..YESTERDAY I FORWARED ALL THE PROOF BUT NO ONE HAS GOTTEN BACK TO ME,but still not help this is fustrating this people are making my life hell!!!!! WHAT DO I DO