Aswath believes, based on some rather cloudy statements in a CommsDesign interview, that Skype has built a proprietary codec that gives them better sound quality than POTS or other VoIP.
Personally, I'm skeptical. For one thing, codec design isn't easy. For another thing, there are a bunch of perfectly good wideband codecs out there (GIPS iSAC, which Skype is known to use; G.722, which goes back to the eighties; G.722.1 and G.722.2, more modern standard wideband codecs (G.722.2 is also known as GSM AMR-WB), though there may be licensing issues), so why reinvent the wheel?
Furthermore, the Global IP Sound VoiceEngine package comes with iSAC (known to be used by Skype), iLBC (ditto), G.711 (mentioned in the CommsDesign article), and with G.729 as an optional component (known to be used for SkypeIn and SkypeOut). If you're licensing a package with that complete set of codecs, why take the time and energy to build a brand-new wideband codec?
Then again, maybe there are a bunch of really bright Estonian speech processing gurus working for Skype. (Speech processing gurus who are Estonian, not gurus at processing Estonian speech...)