I paid my account they debit me for the full amount and want me to pay double

I missed my debit in March from telkom mobile I then received threatening messages saying my account with be suspended in April, I made a payment of R1500 on the 9th of April and the call centre confirmed they received it when I phoned on the 12th of April. I then got debited for almost R2000 when I already paid the account and was only due to pay about R450 now i recieved my bill and it shows that i never paid the R450 plus the other amount that was debited when in actually fact they owe me not the other way around and i dont have to pay anything for may month at all.