incorrectly billed now being harassed

I have received two sms's from Telkom stating that I am overdue on my mobile account. I had a credit balance with telkom which they credited me for on my other contract due to telkom overcharging me for a contract that was cancelled. That credit was used for my existing contract. Hence Jan bill I didn't have to pay because of the credit. Feb I only had to pay a portion of the amount owing on my contract because I was still in a credit. Telkom have not carried all the credits over to the new bill, which has resulted in the bill reflecting as overdue. Please sort this out. My account is not overdue. I do not owe you anything. I refuse to be harassed with threatening sms's thay my service will be suspended.