Billing among clarity

I app**** for Telkom Fiber,the price was advertised at R499 per month.The installation took place second week of May. I received my 1st bill 1st week of June for an amount of R1257.I called umbef 10210 there was no answer to query the amount, I then called 10213 and I was advised to call 10210 I explained that mk one answers that line then I was given an email address to raise my query to date mo knd has answered my email Fearing that they might deactivate the connection due to non payment I called 10210 I waited almost an hour before the line can be answred, the lady answered and explained that I'm billed in advance. Fairt enough meaning my bill was supposed to be for May and June meaning 2 X R499 which gives R1000 then why billed R1257 ow I have received a bill for R648,I need to know how this amounts are calculated.Taking the R499 I app**** for and what I'm billed. I never received a quotation for my application only invoices after installation. Telkom need to explain my billing and the R499 package I app**** for