In the first class I took that talked about dealing with dangerous patients, or dangerous situations in EMS, it was suggested that we come up with a "code" word to use if the situation was turning bad. The theory was that we might not want to make it obvious to the other people on scene, but we wanted a way to clue in our crew that we needed to get the heck out of there, or, if we could not, we needed someone to attempt to get help from the outside.
The code that they suggested was to say something like "Can you go to the ambulance and get the purple bag?" when there is no such bag, and everyone on the crew is aware of that. What it really means, is to get outside, get to the truck, and radio for help. At the very least, it's a way to make it clear to the rest of the crew that you think something is going terribly, terribly wrong.
Another suggestion was that if you were in a situation you could not get out of, but had a radio, that you could use it to try to get help. Turn the volume all the way down, so you (and the hostile person or persons) can't hear anything someone might say, and key the microphone and hold it open. Then, find a way to include information in your conversation with the people. Try to mention their names, your own name, where you are, etc. If there is a weapon present, and it can be mentioned in conversation, do that. Hopefully, someone on the other side of the radio- dispatch, perhaps- will hear you, and be able to figure out who and where you are, and what the problem is, and get help on the way. This might be a good tactic in some situations, but here, the digital radios make that loud noise after being keyed, so I'm not sure it would work as well. Maybe if you can cover that up.
I've also heard a story about a firefighter near here who once asked dispatch very pointedly "Is EVERYONE coming to this call?" until they figured out he was asking for law enforcement back up, but was in a situation where he couldn't say that without aggravating things.
It's good to have a variety of ideas in your head. That way, you might be able to bring one to mind if you need it, and since there is no way to predict what might happen, you also can't plan ahead much detail for what you'll need to do to get out of trouble.
In firefighting, there is the Mayday call. It's pretty standard now, and departments train on it.
As far as I know, there is no such standard concept or training for EMS. I would think that in some situations, it might transfer over perfectly well. "Mayday, Mayday, Mayday, we're on highway 17 near exit 42, our ambulance has just been hit by a tractor trailer and overturned. There are injuries, send help."
Yes, we could also just radio the information, BUT, that repeated, rhythmic, "mayday, mayday, mayday" is VERY easy to catch on the radio and respond to. I heard a firefighter call a Mayday not too long ago, and although most everything else on the radio was garbled and barely understandable, that rhythm was UNMISTAKABLE. It totally jumped out at me and got my attention. So if it's possible to do, without making your situation worse, I think it would be a good choice for EMS, as well. I don't know of anyone who is teaching it that way.
I'd love to hear other ideas, especially things people have actually done that got them out of trouble.