Lyman Frank Baum described what was essentially a mobile phone in his 1914 Oz book “Tik-Tok of Oz”.
“…Shaggy suspected the truth, and believing that Ozma was now taking an interest in the party he drew from his pocket a tiny instrument which he placed against his ear.
Ozma, observing this action in her Magic Picture, at once caught up a similar instrument from a table beside her and held it to her own ear. The two instruments recorded the same delicate vibrations of sound and formed a wireless telephone, an invention of the Wizard. Those separated by any distance were thus enabled to converse together with perfect ease and without any wire connection.
I was thinking about that exact strip. It’s from March 3rd 1974, and Snoopy does a lot of loud barking in it.