We used quite a simple system but it could still get busy at times! Especially as the vast majority of communication was all going on in the same room at the same time. Obviously in the real world the Signalman only hears what's being belled to him, not everything that's going on over the whole line. There were lots of variations in the codes used across the UK. I think ours were based on Southern Railway or possibly BR Southern Region practice, but I might be wrong.
One thing we did, which apparently is quite common, is to reverse the code of certain branch trains compared to the main line ones. So if I got a 2-3 I knew in advance it was a freight for a specific route, so I could offer it on to the right destination.
There was a very old, framed, notice from a real signal box up on one wall, which gave all the elaborate emergency codes for one of the pre-grouping companies. So things like "Train running away on wrong line". It was either London & South Western Railway, or London, Brighton & South Coast Railway, but I cannot remember which. Sadly we never had any need to use or remember those codes and it belonged to one of Dad's friends, so I can't quote from it now.