According to this site, GSM 1900. According to Wikipedia, Movistar uses GSM 850/1900. Therefore, if you are coming from the USA, chances are very good that your phone will work (especially if you have a multi-band phone, which are very common these days). Since there are four main frequency bands in GSM, if you have a quad-band phone (e.g., the iPhone), it will likely work everywhere in the world. As Rory mentioned in the other answer, if you want to actually get a Claro or Movistar SIM card, you are going to need to have an unlocked phone. If, on the other hand, your current provider has an international roaming plan, your phone should work fine. If you plan on taking the latter option, I suggest you call up your current provider, tell them the model of your phone, and they should be able to tell you if it will work in Nicaragua and how much your calls will cost.