I was reading the wiki, and I wonder -- did Heinlein "invent" the cell phone? Twice there are mobile phones in his juveniles -- once in Between Planets and once in Space Cadet.

Is this the earliest mention of such?

You missed the earlier mention in "Waldo."

I don't know if Heinlein was first in this respect - it's such a obvious idea I suspect a search of early sf would turn up several precursors - but if you reread the passages about Bill and Matt each being embarrassed talking to their fathers in front of friends, and Tex jovially having packed his phone away to avoid being bothered... he might have been the first to think the sociological implications through.

You have wiki-whacking privileges, you know. Use them!