Down-payment

Someone explain why I have autopay. Been a customer for over 7 years and now that there is a promo for a $5.00 a month phone so I wanted to add a line,  it says I have to not only pay down-payment but the cost of phone would be $18.00 plus a month not $5.00 due to my credit. I do not understand this at all since our credit score is nearly 800. I was chatting with someone online and they told me to call customer service.  I did and it said the same thing to them and could not give me a reason for this. I either pay downloading and increase price of phone or don't add a line.  Talk about false advertisement.