Using your 'foreign' cellphone in the Philippines can incur huge roaming charges. Sending an SMS text message can cost one or two dollars each. Making local calls will be about $2 per minute.
If you have an unlocked cellphone, you can simply buy a local SIM card to make it local.
Alternately, for about 2000 peso (~$45), you can purchase a local cellphone (Nokia 1110i unlocked, including SIM card and P300 prepay credit). Local SMS text messages are 1 peso each and calls are 7.5 peso per minute.