Incorrectly Billed.

Telkom sends an invoice every month. Each month there are new amounts, which get bigger. The amount that appears to be due at the end of November is beyond the amount Telkom and I agreed on. The amount we agreed I will pay monthly for my data bundles is R99.00. Now that amount does not even appear on the invoice. I am so upset. I refuse to chase after them by calling them. Please assist me by getting them to call me by tomorrow latest. I need to understand once and for all how much I need t pay exactly, and where the other amounts come from and how exactly their calculations build up to the amounts I see on their invoice.