Under a new law you need to register your prepaid sim card before 18th October which is designed to help you if your phone gets stolen.
I've been on prepaid/top-up for years where you buy phone credit at a kiosk or other places, but a year ago moved over to a monthly plan (the cheapest) with Movistar since prepaid really is a rip-off.
Anyway, if you're still on prepaid you need to register the line or you'll get cut off.
Hit *234# and enter your document number (id card) and that's it apparently.