Double billing

I took out a new contract. I only received my phone on the 24th of august, and only activated it on the 06/09/2017. Now i get an invoice where i get billed from the 1st of aug to the 31st of aug. But i only received the phone on the 24th... how does that work? How can i pay for a full month if I only received the device on the 24th? When i took out this contract i was told that i would pay pro rata which i dont have a problem with. But to get a bill that is double the amount that you were told is not right. Please have someone sort this out.