It's not just the μ-law backend, though; it's also the codecs used on the cellphone network. For example, 3G GSM supports AMR-WB, which is apparently fairly good quality, but the carriers don't have to enable it. Some European carriers use it; but they have actual competition. I'm pretty sure AT&T, for one, limits their network to AMR, at half the bitrate.

FaceTime can do better because Apple is bypassing the carrier network altogether.