A hangover from the old VHF radio system, each SES vehicle has a unique three digit number. For example Prospect SES use 442 and 441 for their vehicles and Metro South SES have 331, 332, 333, etc.
Unlike CFS, the vehicle numbers have no relevance to what the vehicle is or its capability. But it makes for short radio transmissions and minimal confusion at a multi-unit incident.
As Mike stated, to avoid a persons name being transmitted each officer has a number allocated. Some Units use this often, some do not. For example, State2 is a certain person.
If you are using a GRN handheld radio, then it is allocated the callsign of the vehicle the crew are associated with then Alpha, Bravo, etc. So Prospect SES were using 442 alpha and 442 charlie at the Glendie Greek festival display on the weekend.
The use of broadcasting time by some SES Units is a local Unit operational requirement. Some SES Units have voice recorders on the LHQ base radios, so maybe if they state the date/time of the call during the radio transmission then this is recorded. It is also a quick way for Unit officers to keep track of the response times and how long between radio welfare checks. I am not absolutely sure of the reason because we do not do this in out SES Unit.
I hope this helps.