These things have huge resolution now and the user can pick their language; would it really kill them to write words over the little icons? If in English, perhaps "ANSWER", "HANG UP", "MUTE" and so on, but the magic of words is that it doesn't have to be those exact words ("HANG UP" and "END CALL" are completely different sets of words yet in the context of a phone call, mean the same thing to almost everyone - magic).
Words. They carry so much information. So much. I know, people want to find some magic picture that carries full meaning to all cultures, but there simply ain't no such picture and there never will be; there's no shame in using words. Please. Use words. I'd even happily take them in a language I don't even speak, so long as it used an alphabet I could read (or even not - I can read some common software related words in Japanese simply through having sounded them out a few times). For me at least, words are easy; the ever-changing mist of icon-style-du-jour is not.