Siri not sending text / sms

I cannot get Siri to send a text/sms. It allows me to compose the message, confirms the recipient and the message, and then tells me that "something is wrong, could you please try again." Strangely, sometimes (though not often) it actually sends the text but still informs me it can't. I am in Western Australia on the Optus network.

I have tried turning Siri off and on, resetting phone, turning wifi off blah blah.

Siri's other funtions work fine, such as taking notes, reminders, alarms, weather, placing calls, telling lame jokes.