Morse sender and receiver for line transmission
Hmm, if you are making it simple then I suggest the following circuit for each station.  They will all be the same and connected in parallel up to the supply and cable loss limits that you have.

All devices on the bus will be signalled and your own buzzer will not sound.

EDIT:
The LED can draw from 5 to 15mA (visible in the dark, or daytime), the Buzzer from 1 to 20 mA (Piezo or electromechanical) so with the regulator quiescent current (say 1mA) you could hope to keep it under 20mA total per station.  

If you use alkaline batteries intermittently you could expect to supply 100 to 500 mA current, allowing a maximum of 5 to 25 stations depending on cabling and battery quality. 

With 6 stations each station could transmit for about 5 hours (500mAh capacity) or a total of 30 hours of buzzing (at 100% duty cycle many days in practice), that is a very long field exercise, the scout master would go nuts from the noise before the batteries failed.  With 26 stations (using good quality batteries) you would be limited to 1 hour of buzzing by each station, that is still a long field exercise.

The batteries would fail first in the station that transmits the most, if it was HQ then they could keep a spare battery on hand or use a larger one.  Other stations might go for a long time with standard small PP3 alkaline type if they make shorter responses.  

When a battery fails you only loose the transmit capability and can still follow a recall order or make arrangements to series connect batteries with adjacent stations to double voltage if running low. If the LED was connected before the voltage regulator every station would be able to visually gauge the relative battery capacity and report on this (a small moving coil meter could be used instead) to practice signal strength reports, with distance units or those with depleted battery capacity  providing a lower signal. Placing the LED onto the common point of the switch would let each station monitor themselves and their own battery status and shorts on the line. A mute switch for the buzzer would allow for operation in the quiet.

Many automotive grade voltage regulators will be hardened against reverse voltage (bus or battery), over temp, short circuit, over voltage, some static discharge and more likely to handle field conditions better than a transistor as a first line of defence to the outside world.







simulate this circuit &ndash; Schematic created using CircuitLab

Modified schematic with monitoring:







simulate this circuit