Sending international SMS on Movistar prepaid SIM (Santiago de Chile)

I have a Movistar prepaid 4G SIM which I have loaded up with credit to allow calls, SMS and data. I can make and receive international calls (albeit having to enter Protected content of '+'), and can receive SMS from Australia, but my various attempts to send an SMS to Australia have all failed to get through. Similarly if I reply to an SMS from Australia it doesn't work either. I don't get an error message, it simply never arrives. Do others have this problem? Does it work on other networks? Is there a way of making it work?