You will usually collect your radio each day from the 2 AD / Production
Check the channel you are supposed to use - often it will be the production channel so you will know what's going on, other times you may have your own or be shared with Health and Safety teams.
Usually Unit Medics - work on the ADs / Production Channel
Construction Medics - work on locations channels (usually where no filming is taken place, but areas are set up or rigged)
Radios tend to be on silent mode by default - it will not beep or make noises unnecessarily, therefore they often will not even make a noise when you turn it on.
Ear pieces are mandatory on set, keep at the lowest volume, but still so you can hear the messages
There are no formal radio protocols, other than once you've heard "TURNOVER" don't use it, until you've head "CUT"
If they need you, they will shout either "FIRST AID", "MEDIC", "UNIT MEDIC", "your name" etc