most people who were in the military before the internet and cell phones were invented have been in a big phone center, somewhere, but I bet few were on a phone bus!

The New England Telephone Company had a fleet of buses with phone banks inside that could be connected to local phone lines. These “telebuses” drove to wherever they were needed, hooked up their phones, and welcomed callers to come onboard. 

this one is at Naval Training Station Newport RI during World War II

Telebuses survived long after the war and were used at the Newport Jazz Festival as late as 1957.

https://navalwarcollegemuseum.blogspot.com/2016/05/the-telebus-comes-to-naval-training.html