Should a phone represent a call? How do we avoid the trap of keeping a visual metaphor, like the floppy disk for saving a file, as more and more of our physical tools are digitized?
Sometimes yes, sometimes no. A cassette tape represents music well if we're talking about, for instance, mixtapes, rare recordings, bedroom bands, and indie music.
Things like phone calls (which in 2012 really means "audio calls over great distances using some kind of device") and music and messages and saving don't have any visual representation since they're not visual ideas. So, it's natural to want to visually represent them using actual, visually rich images associated with them. That's what metaphors are; there are no birds or stones in most situations people apply "two birds with one stone" to, and yet the expression is rich and meaningful.