You could make the phones communicate via sound, not sure how it would impact the battery nor its communication range. Lots of details would have to be worked out. You have to find the frequency that communicates the longest. Perhaps do short pulses of sound to save battery life.

Unfortunately bass travels the furthest. I can hear cows a lot further away than frogs, and I can hear transport helicopters for miles (miles!)

The problem with bass sounds is ostensibly they're nondirectional. So the ideal sound for a phone would probably be the loudest pink noise they can make without blowing the speaker out.