Ignorance MTN

I took a contract phone with MTN and I've made a debit order of the 15 of every month for R409 it happened Feb 2020 they went on the 20 Feb as there was no funds and my phone was suspended as I didn't know the stop order didn't went through I went to complain as why my phone wasn't working they explained that I didn't pay for the month than I've made the arrangements for the to debit twice on the of March of which they didn't I went I've called and ask for their account number n references and I went and pay cash at Absa R839. 50 for Feb and March I've e-mailed the proof of payment and phone was open the same they debited R1200+ I went to bank a reverse the and pay R409 for April as the phone was again suspended and after paying do the same process of emailing, the month of May R1600+still was debited I went again to do the reversal and went to pay cash again, I went to their shop to lay a complain and the who was assisting me forwarded my complain twice but still I didn't get help, now MTN is threatening me with emails saying they will take legal action against me and I'm not owing them nothing as I've still in my possession of proof of payment Complain ref number SR#*** and SR#1-*** and Ref num for 4 payments BA*** I'm sitting with non working phone