Calling for <...>

Usually, you see the Cloud with Red +, if you're playing Medic and somebody's calling you.

Now, about other possible Callings:
1) <<FIRE!>> (Cloud with the icon of the Bonfire inside of it)
There are some classes that can extinguish their teammates. If you're screaming <<FIRE!>>, they could see it and help you. Also, making the position of Pyro noticeable.
2) <<HELP!>> (Cloud with the icon <<!>> inside of it)
So, anybody who turn this option ON (in options) will see your callings for help.

/to be continued.../