For the passing of simple pre arranged messages, rather than 2 way voice contact, dont forget such basics as colored flags or similar by day, and colored lamps at night.
In clear weather ranges of some miles are easily achieved.

This can be a usefull suplement to cellphones, walkie talkies, or ham radio if battery power is very short.
A simple pre-arranged code could save scarce battery power.

RED-------------turn on radio or phone ASAP and make contact
YELLOW----------radio/phone defective.
GREEN-----------All is well, no need to consume battery power.
BLUE------------Make contact briefly at 18-00 local time.

Or some other system as required by local conditions.

A coloured lamp need not be electric, oil lamps for railway purposes are still available and give a red, green or white light.
A single 5mm LED consumes less than 0.1 watt and can be seen for miles if correctly aligned.
A flag of course requires no power or fuel.