As far as I understand, Skype is better that straightforward VoIP provider because it uses the technique of P2P in order to transport the packet carrying voice, rather than QoS which become too expensive in the long run and is dependent on the provider's maintenance and configuration.

Is this advantage borne out in real life? I personally experienced VERY good voice quality using Skype, even when running via an old laptop of 400Mhz, although I have nothing to compare with (other than IDD which is not fair), does anyone has good experience of other VoIP provider to compare with ? (I am only interested in voice quality)

I ma not really getting... Normally, voice is not transmitted via other clients; it travels directly peer-to-peer. It is the same with other VoIP providers, at least with the majority that employs the SIP protocol. As far as I have understood, other clients act in a similar way as a NAT server does in SIP environment.

On the other hand, QoS is a technology of assigning priority to certain types of data on a network. It relates to a different protocol layer than the concept of "P2P", and I don't think you can compare the two at all.

Skype has neither a better voice quality or speed or anything else than other VoIP solutions. What P2P technology does, is (1) removing the need for a central server for the communication between the clients (other than authenticating them), (2) making Skype function behind most firewalls (reliability!).

And you have other pluses of Skype: text chat, video, and finally good marketing...

I was wrong in my previous post. Actually, Skype packets - including media (voice, video) - can be relayed via other Skype clients ("supernodes"). In SIP protocol it is not possible - media always travel peer-to-peer.

(http://www.secdev.org/conf/skype_BHEU06.pdf, pp. 98-100)

I read somewhere about scalability etc, because of this technique, other VoIP relies on the supplier actively monitoring and adjusting their network to ensure that it is modelled to the call pattern, which also mean that they will deliberately leave some trunk oversubscribed,i.e. bad call quality, for economic reason. Thus profitable route will always have excellent call quality so as to keep the customer whereas lesser route may be neglected, Skype suffer no such limitation and Skype does not own the network (I might be wrong) nor rely on QoS (you can only run QoS on your own network, not on the Internet), I read somewhere that SKype can thus scale up indefinitely and in fact the more people use it, the better it become !!

You seem to be talking about some big companies with heavily overloaded networks and internal VoIP communication.

However, most VoIP providers we would talk about are public providers, that is, which anyone with Internet access can subscribe to. They don't own any network, and their servers only handle users' authentication and possibly PSTN ("Skype-Out") calls. Similarly as in Skype, the media (= voice) travels on the public Internet where QoS can't be enforced. The only advantage of Skype I know about is its ability to work behind any firewall.

As regards calls to fixed telephone networks, Skype uses exactly the same range of independent providers for termination in each country as other VoIP companies. These providers are sometimes "better", sometimes "worse", which remains beyond any control of Skype. Skype-out calls to Pakistan are of much worse quality than made through, for example, Voipfone - and this has nothing to do with Skype technology but with the provider they have chosen to use in that country.