my account is sent to lawyers without a reason

I upgraded my contract with cell c direct but I received a wrong phone and i returned it within a week. it took them a whole month to attend to my matter after thousand calls I made, eventually I got my phone but i could not activate my Sim card because they said i owed for one month (the one month i did not pay because returned their phone). After that I paid my installment as per agreement every month and out of the blue i got a statement saying R6144.0 I called cell c but I did not receive any help then i sent a complaint via email. someone returned my complaint and they reversed that amount but it kept on appearing on my statement and credited/reversed every month. Then out of the no where in August I received a notice from Debt collectors( SSD) on behalf of Cell C, saying I owe their client R 5815.21. I tried to resolve this matter but i had no luck, no one is able to help me.<br> I really need to resolve this matter asap because I pay my installments every month. I just need an explination