Or, and hear me out, you could say “portable fax” and be done with it. YOU are making it complicated by not being culturally acclimated to the timeframe when it was written. Everyone knew what faxes were, no explanation was necessary.
Portable fax: thing that sends and receives messages
Portable Fax IS how you describe SMS in the 80s.
I dont mean that your understanding is unimportant, but that you inherently understand what’s being described to a degree that to hear it described differently than you expect you reject what you hear in favor of assuming the folks in the 80s needed more than “portable fax” to understand what you are on about.