Cannot send text to international numbers

I bought the Iphone 4s about a week ago, before that i was using blackberry. I realised that i couldnt text anyone internationally since day one and had called my provider (Telkomsel) and they said there's nothing wrong with their network. I had to believe them since there was never any problem with my blackberry before when it comes to texting anyone, anywhere.

I can text everyone locally (indonesia) just fine, but why couldnt i text me friends overseas? I ve put the + sign in front of their numbers, make sure i had the correct extesions, tried turning the international assistance on and off to no avail. Can anyone help me?