The Global System for Mobile Communications frequency used in the Philippines is 900 or 1800 MHz. The United States uses 850 or 1900 MHz. To use a U.S.-based mobile phone in the Philippines, it will have to be a phone called a quad-band or "world" phone. These phones have the capability to switch from domestic to international frequencies. If you do not have such a phone, you can call your domestic provider to upgrade. Note that you may need to pay out of pocket or extend a contact to obtain such a phone.
Originally, the Philippines used analog cellular services in the 800 MHz range. Although GSM is more prevalent, you will still be able to get service on Indonesia's Time Division Multiple Access network if you currently have cell phone service with AT&T or Cingular in the United States.
World Phone Rental
If you do not have a long-term need for a mobile phone that will work in the Philippines, you can rent a world phone from places like Mobal.com, CellHire.com or RentCell.com. The fees incurred are often less than the charge to purchase a phone from your carrier.
You can purchase a prepaid phone while in the Philippines or online before you leave for your trip. The rates for prepaid service are usually less than the international roaming rates charged by U.S. service providers. One drawback of this option is that you will have a Philippines phone number, so anyone in the United Stated will be charged international dialing rates to call you and many data services, like email, will not work.
If you plan to visit remote locations where standard GSM or TDMA service will not be feasible, you have the option of activating satellite phone services (see the Resources section). Anyone who needs such a service will be required to pay a high per-minute price.