Custom ROM dev and Networks How are International ROMs made?



I have downloaded and installed a little while ago a ROM which is the best I've ever used in terms of speed, smoothness, battery life, and customization and features. However, I am unable to make calls, send SMS and MMS and use the Internet (Mobile Data) due to the fact that I am in Australia and the ROM is not an International ROM.

I am a developer. However my experience in this regard is limited. How would I go about making this ROM support Australian networks? If you know anything at all I would appreciate your help on this. Even if its something as little as a keyword to search for because I have been searching for a long time and have not found a single result that was related.
There's no such thing as an international ROM. When you see (for example) a Cyanogenmod ROM "Samsung Galaxy S III International", it means that the ROM is for the international variant of the phone hardware, with the right drivers for that phone's hardware. The international variant might just have a different radio (to work on the right GSM frequency for your country), but sometimes the international variant is a completely different phone, with a different SoC or screen, but in the same case.

If your phone is an "international" variant, then a ROM that's for the US variant will be missing drivers you need. Fixing this problem requires finding the correct drivers for your hardware and integrating them into the ROM.

If there is only one variant of your phone, or the ROM is already for the correct variant, then you don't need an "international ROM". The only important difference between ROMs for different regions (in this case) is that they have network settings for networks in that region. You just need to get the right APN settings from your network operator (you can probably find them on their website), go into the Mobile networks settings in your phone, and input the correct settings.