Billed after settling account

I settled my cellphone account by obtaining a valid settlement letter at one of your branches because i could not handle one more second of the contract i had that was all lies from the start . Finally when I get rid of you, I get bills saying I still owe you. I really am t my witts end and i sincerely hope my credit record has not been tarnished. I specifically remember my phone moving over to prepaid yet I GET BILLED!!! FOR WHAT!!!!??? Please have someone call me and sort this out I am beyond furious because I settled the account in full as per settlement letter.