not sure myself, I'm way out of my depth here, but a friend who knows much more about this sort of stuff, told me it's probably something to do with mobile connections. Apparently slow landline connections and bad cellular connections like 3G are unreliable in different ways. Also most mobile phones, definitely all iPhones, not sure about Android, can't go higher than 16kHz when it comes to audio calls, so anything better is pointless. Maybe it's just M$ and trying to save some pennies on the infra, especially that they're not p2p any more (also b/c mobile connections and restrictive NATS). I know They're supposedly not that bad any more, but considering what they've done to Skype's other aspects, I wouldn't be surprised really.