I am subscribed to Smart Communications post-paid service. For activation of the international roaming feature, I paid a PhP 2,500 deposit which I can later claim if I choose to deactivate the service. Let me take you to a few screen captures as my phone roamed the international GSM network. Some networks are 3G while some are not.
South Korea - I guess SKT means South Korea Telecoms
In Japan my phone roamed between Vodaphone and NTT DoCoMo
Telcel in Mexico