Incorrect billing and **********ly invoicing clients

I have been harassed by cellc for months now. Their managers and agents always pretend to help ans never do. I had 2 contracts with cellc which both were due to end in 2020 December, i upgraded 1 contract and let the other 1 to run on a month to month. In March i called cellc to cancel the one ln month to month and they advised of the notice, i served the notice in the month of March and paid the March invoice and then cellc due to negligence they didn't canceled the contract and let it run to April. Now they billed me because they didn't cancel the contract. I reported and spoke to many team leaders until i spoke to the supervisor who assured me she will fix it. On May 2021 the supervisor that works in sage building called me on the canceled line of *** and told my girlfriend not to pay the money they charged in error and also spoke to me on my *** that I must not pay them she will handle the matter and she further more mislead me and told me to pay only 433 for my upgraded line i did that for a couple of months then November and December inam being called about arrears, because of the line they canceled late and i have been under paying the upgraded line while I paid what i was told by their manager. When i escalated this matter here they respond and say they will contact me and never contact me and they never resolve the issue. You can check my other complaints about the same case, they have not resolved this matter in over a year and they messing up my credit score. I regret joining cellc. I am never taking anything with them ever. They ********** and ***** from clients. They are liers