Selling lies

I am a cell c contract holder I took a router contract December (2018) last year over the phone the lady who was selling this contract told me its promotion monthly installment is going to be R289 for 40 GB but I will start paying end of January 2019 but for December I am going to pay small amount as R50 I asked what is that R50 for I didn't get clear answer few days after I have received a router boom R188 is debited on my account even worse it was even before my pay day okay I ignore that thinking maybe its her "R50" what make me mad is that when I received my statement for January money due is R811 where its comes from, I called cell c they say they gave me router and with R500 airtime so I finished my data and use R500 airtime so my questions why I was not told about that R500 and on my contract there is nothing that talk about R500 airtime why are they selling lies?