fono

Fono is what something stands out against: the plain surface behind the subject. A green star on a white fono is the Esperanto flag. It works figuratively too, for anything present but not attended to - fona muziko is background music, fona bruo is background noise, and a computer task running quietly is in fona reĝimo. The opposite is malfono, the foreground.

Now the trap, and it deserves untangling honestly rather than in one line. You already know telefono, and you can see fono sitting at the end of it. So: is a telephone a "distant background"?

No, and the reason is that two different words arrived in Esperanto looking identical.

• The -fon- inside telefono is Greek phone, sound. It is the same piece as in fonetiko, mikrofono, saksofono, simfonio. Telephone means far-sound.
• The standalone fono on this page is French fond, the ground or bottom of a thing, from Latin fundus - the same root as English fund and foundation.

Same four letters, unrelated histories, and Esperanto did not merge them. This is the sharp edge of the affix-hunting habit: when a familiar shape turns up inside a long borrowed word, it is sometimes the piece you know and sometimes a different piece wearing the same clothes. Telefono is one indivisible root in modern Esperanto anyway - you cannot take a tele- off it and get a word - so the question never has to be answered in practice. But it is worth having answered once.