My employer had a security notice on its reception desk and that too, called the instruments correctly, portable telephones.
The wrong word probably comes from some big-name advertising company, and relied on enough people having not been taught to understand ordinary words in their own language!
I use "portable" and once puzzled someone enough for him to ask, "Don't you mean 'mobile' " ?
"No. Mine isn't mobile" , I replied. "It has neither motor nor wheels."