Large amounts of data can be transported over dozens of kilometers using modern fiber optic cables with little signal loss. With the right selection, the glass fiber also offers increased protection against lightning damage, moisture, mud, etc. since these are made of plastic. In addition, it is bug-proof - for whom that would be important.
Modern communication facilities in tunnels and mines are based on a head station and are connected via fiber optic cables with sub-distributors, from where the usual modern communication devices can be connected. A cable connection can be used sensibly, especially in underground rooms such as caves or tunnels.
Speleologists around the world use a single wire telephone. These small pocket devices can transmit voice signals for miles over a single copper wire.
The same, but with a lower signal bandwidth, can also be achieved with copper cables (telephone lines). Since metal is known to be a good conductor of electricity, overcurrent b.z.w. Surge arresters should be considered as protection for the connected electronics.
If only one voice connection is required, old field telephones can be used sensibly. The range for such a field phone (crank phone) is under favorable conditions (good cable, no interference, optimal cable routing) up to 15 km in civil engineering (laying on the ground) and up to 40 km in building construction (laying over masts, trees, Etc.). The range of used technology is large.
When laying cables, care must of course be taken to ensure that they cannot be accidentally cut or torn off.