I've installed SIP phone systems using CISCO and POLYCOM handsets, and the call quality is first-class, better than a classic analogue call, while the same PBX using a Lynksys SPA phone makes the caller sound like Darth Vader with a head-cold.
We have Polycom phones in our offices (running what looks like Windows CE, no-less). They're IP phones that are integrated with Lync, it's quite cool to see Lync on my desktop interact with my handset, but I've found the call quality to be no-better than a POTS call.
I did have an audio-only Skype call with my mother (me using my iPhone 4S and my mum using a Nokia 820) and the call quality was... like nothing I've heard before, even when using Skype on my computer. I guess the 820 has a top-quality microphone.