Suppose I take my phone on roaming from one country (say Australia) to another (say Belgium). How do these plans typically work? Will somebody else in Belgium have to pay international rates to call me, given that they would have to call me using the international prefix? Will I have to pay a roaming fee to receive the call?
The person calling you will be calling your Australian number, so they will be charged whatever they normally pay for a call to Australia. As you are roaming, you will be charged for receiving the call at whatever your roaming rate is.
In most cases, the call itself will actually route via Australia and back to wherever you are! It's possible that if your phone is roaming onto the same network as the person that's calling you is on that the call will route locally, but there's no guarantee.
The reverse case is very different. If you call a number in the country that you are in then you will normally pay a lower rate to make the call than it costs for you to receive a call, and the person you call will only pay their standard rate for an incoming call (which is free is many/most countries). Obviously the exact costs will depend upon your provider and what they charge for calls whilst roaming, but in most cases it'll be cheaper for you to call someone in-country than for them to call you.
Depending on how long you're going to be there, the best option is often to buy a pre-paid SIM in the country your visiting. You'll get a local country number that people can call you on, and you won't pay high roaming costs for calls that you make.
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.