Rude agent and bil doesn’t make sense

I have just opened a contract at mtn. My bill says R500 but i did not go over my spending limit and the contract is for R300 a month. Phone then the agent i speak to is very rude and impatient. Tells me that they can’t tell me what my bill is till february..how is that even possible?? It should be R300 not more then that when i just started the contract and my spending limit shows R0