This is true for calls inside your home network. When you place a call to an on-net device, the network knows almost instantaneously if you're available or not.

This is true for calls inside your home network. When you place a call to an on-net device, the network knows almost instantaneously if you're available or not. Let's say you're roaming: how does your home network know how to reach you? Without going into a lot of detail, that information has to be relayed from your roaming carrier to your core carrier. Most likely, the ringing you hear is the border of the home network waiting for a response from the last registered roaming partner network. As in all cases, every tone you hear on the network these days is just an arbitrary audio file and has nothing to do with network operation today. Phones ringing, busy tones, etc. are all relics held over from the old days to make people feel better and more comfortable.