I believe you need a permanent DNI to get a mobile phone plan. In the Movistar account page, can't you download your itemized mobile usage with your name and address?
There are some advantages to being post-paid, you might get bigger data packages as well as other benefits, e.g. Flow with Personal, basic connectivity when your data allowance is used up (i.e. lower speed, rather than nothing), also mobile operators may differentiate service to pre- and post-paid users if the network is congested, etc.