We are having this exact same problem. Three members of our family, all on Virgin, all different phone (only one Smart) unable to send messages to my sister in Australia. We can receive her messages fine but unable to send.
We are having this problem too (three members of the family, all different phones). Have just spent ages on phone to 789. She checked everything to make sure nothing barred etc and that there had been no updates regarding change to service. The only thing she could suggest was that my sim is outdated (3G) so is sending out a new sim. Really annoying to suddenly not be able to text Australia. If you haven't already called them, please set aside 40 or so minutes to call 789 and let them know you're having the same issues. They said they hadn't had any other complaints or seen anything on the forum. 😕
Both my other half and I were trying all day yesterday. My other half called 789 and they went through everything with her and then told her her phone was broken and have arranged for her to return it to them for repair, this will leave her with no phone as they aren't willing to send a replacement. The phone is less than six months old.
I'm not sure what we can do now, it's clearly a Virgin issue but getting them to acknowledge that could prove problematic.
I'm sorry to hear of the issues faced. It's a strange one as I've never really heard of an issue like this regarding Australia apart from this thread, so I'm eager to make sure we can at least work out what's going on. When you try and send the message etc, do you get an error message or anything of the sort?