iphone 4 no service!!!!

My iphone 4 is from canada. Factory unlocked. But when my grandfather sent it to me here in the philippines, i inserted a smart buddy sim it says that 'searching' and then 'no service'. But after a day i had a cignal. It only last 10 mins and then it went back to no service. It just keeps going and hoing like that. Ehat seems to be the proble. Help!!!!