Apparently the answers lies in changing the message center number on the iPhone!
Go to the keypad on the phone and type this:
DST Brunei: **5005*7672*+6738795000#
bmobile Brunei: **5005*7672*+6738101010#
Telstra Australia: **5005*7672*+61418706700#
Type the above number into your keypad and call it. Now it should be set.
Useful tip for anyone in Brunei having this problem.