Simple options:
1) If you want a phone to use in other countries as well as here...buy a mid to high end new or newish UNLOCKED QUADBAND GSM phone on ebay or elsewhere. It will usually state at the beginning of the description GSM QUADBAND... for the record, last I checked Verizon is NOT GSM...they are still CDMA, along with Sprint. At&T and TMobile are GSM. When you get here, take the phone and your passport to a claro or Orange store or sattelite shop and buy a sim card for 300 pesos, this comes with 100 pesos of calltime.
2) if it's just to use here, buy a new "cheapie" phone here at any Claro, Orange, TriCom or Viva store. (Claro and Orange are by far the biggest with the best coverage) Right now you can buy a brand new phone with sim card and phone number directly from Claro main offices (and I assume all the sattelite stores) for 595 pesos which includes 100 pesos of calltime..and you only need a passport.
After you have purchase your sim card or new phone, buy a few 200 or 250 peso phone cards...have them show you how to enter it into your phone...then do one yourself, while you're still there. Keep an extra in your wallet...