I think it's something to do with the new airwave system (radio and comms). the army had it about 7 years ago, fire brigade got it a few years back.
The idea is every emergency service can communicate across the country. There are talk groups set up for police, ambulance, fire, coast guard, everything, that is why all callsigns have changed to become nationally unique so there aren't two the same.
Airwave bills the user for every messasge received! I know i know it's bloody stupid! The cost to the public must be immense. At work, when the trucks aren't out we have to turn the radios on to standby mode so they don't receive messages and get charged for them! Silly public sector procurement!
The radios are like mobile phones in that you can send a text. the good(or bad) thing about this is, the sender (boss) can see if the receiver(worker) has received the message or not.
Sending these 'texts' is cheaper than sending a message over the talkgroup as only one specified receiver would receive it, not everyone on the entire talk group, therefore there is only one charge, not hundreds for example.
As airwave is digital and alot more secure it's so much better in these times of increased threat.
I can't remember exactly how it works, but it's something to do with packets. The broadcasts can get broken down into smaller portions, somehow that means, more info can be sent on less!