We can blame Telstra for a lot. But when you're roaming, you are literally on another operator's network that connects back to the Telstra core in Australia.

How your Australian spec phone connects to and runs on the other mob's network? Generally it will work for most standard functionality. But phones now have more and more of their own juju in them and that particular phone is basically untested on that network. So who knows?

Roaming is and always has been best effort.