Germany

It's a pseudo-anglicism, like Oldtimer (antique car), Homeoffice (work from home) and Flipper (pinball machine).

Pseudo-anglicisms arise when a languages lexical composites are known in a non-native population without perfect knowledge of the actual vocabulary. All the words above are build out of perfectly fine english composites, just put together in a way that "feels" english to Germans.

There are also pseudo-germanicisms in english too by the way, the NYT had an article about "Freudenfreude" which was supposed to be a german word with the opposite meaning of Schadenfreude. But while it would be a logical german composite-word, it doesn't exist as such. "Freudenfreude" is only ever found in english literature.

I think because they are handy to have and they fit perfectly into your hand.

Edit: Or maybe from "handset".