nokia 6600 sim card registration failed

i got my nokia 6600 unlock and it worked with t mobile sim as well as using a cingular sim card. but when i tried using a overseas sim card that is roaming internationally (philippine smart sim card) the phone displayed "sim card registration failed" . anyone knows why?

its to do with your provider, give them a ring and make them allow overseas calls and make it so your fone works abroad