Just watched a video about the Ukrainian front [trench] lines in the Donbass region and, in it, the reporter (a USAF vet) explains why Ukranian forward positions use field telephones instead of radios or cell phones. You can probably guess the reasons, but it's explained at about 6:30 in the following clip.
https://www.youtube.com/watch?v=8Flf1grUWCU
That's one pro of field telephones. The big con is that they can be severed by shell fire and vehicle traffic, and tapped by enemy infiltrators. During the Vietnam War, SOG ran missions in Cambodia and N. Vietnam where the objective was to insert listening devices and transmitters in NVA field telephone lines.
On a very tangential note, my wife's grandfather (RIP) was a linesman in the Philippines when the Japanese invaded. He spent four years in Cabanatuan POW camp (and makes an appearance in Hampton Sides' book about the raid to liberate the camp,
Ghost Soldiers).