In its overall descending into a nearly-failing state because of the military power that the drug cartels have built up for years (and that you can't just disarm like that) the Mexican government has decided that it will block all cellphones that aren't registered. Off course what is the use of listening to a cellphone if you don't know who is belongs to ?
I don't think this measure would be very popular because around 30 million phones aren't registered yet..... and saturday is the limit
"Most of Mexico's 84 million mobile phones are prepaid handsets with a limited number of minutes of use that can be easily bought in stores. The phones can be topped up with more minutes through street corner vendors." source
But even afterwards this will not be of much help because there are always ways around.