I went to Movistar a few months ago because they had randomly changed the address on my bill and I needed the correct address for proof of where I live. The lady told me that as of last year, you can only get a postpaid plan with a DNI AND premanent residency. Temporary residency is no longer sufficient to a postpaid plan.
Unless you are a citizen or permenant resident, your only option is the prepaid plan according to the person I spoke with. However, if you already have a postpaid plan you get grandfathered in. You just can't change it.