From FAQ: If you're a Postpaid customer, roaming activation is automatic (except for corporate managed accounts). But in case you previously requested to deactivate your roaming service, you would have to request for reactivation before being able to use the roaming services. In case you fail to reactivate the service before leaving, just have someone from the Philippines call the roaming hotline at +63 (02) 7301212 and have the service activated for you.
If you're a Prepaid customer, all you'll need to do is text GROAM ON to 2884 to have your roaming activated
Remember that you can always call the hotline at least 24 hours before your flight's departure time so you can check whether your roaming service is activated or not.