Not many people knew we had energy weapons in WW2. They came a bit too late, though, and were too weak. At best they gave the enemy a mild sunburn.
The ad was a reminder to keep the phone lines clear, a concept that's mysterious to most people today. You mean there's a finite capacity? If too many people are using the phone, other people can't? That makes no sense.
But so it was, and that's why you were kindly requested - nay, sternly urged - to keep "your own calls as few and brief as possible. And you'll be bring Victory that much nearer."
Through short phone calls.