Are calls to international numbers charged at international rates if the caller is in the same country?

I used to work in mobile phone billing, but that was a long time ago (15 years or so). At the times the rules were simple: Outgoing calls were billed at the rate of the roaming provider + roaming charges. Incoming calls were charged to the person calling you at his local rate to call you at your current location + roaming charges charged to you + international rates from your home country to your current location charged to you.
In your situation, you and the caller being in Belgium with your phone having an Australian SIM, this would mean the person calling you pays the same as if you had a Belgian SIM, but you pay international charges from Australia to Belgium as well as the roaming charges applicable between your Australian network provider and the network provider you're connecting to in Belgium.
As you can imagine, this can get very expensive very quickly, and many users never know this system is in place until they receive cellphone bills to the amount of thousands of Euros.

By now all this may well have changed of course, but at the time it was pretty much standard between operators to charge in this way.