Session Initiation Protocol's (SIP) technical superiority and IETF endorsement.
This is debatable. I worked with SIP and use Skype and it was obvious to me that Skype was winning because of it's technical superiority. SIP was initially designed by people who forgot that most internet users are behind a NAT, so they mistakenly built a protocol that embeds IP adresses. So at first it just pure didn't work, then we saw a rising of various addon protocols like STUN and ICE to figure out whether you're behind a NAT or not, but even then it was just complicated. Later SIP would just compicate itself due to the need of being compatible with the legacy phone network, which made the protocol pretty much unreadable. Oh, and do you want a secure encrypted voice call? That standard is pretty recent, key management nonexistent, just like interoperatibility.
All the while Skype will just bypass any corporate firewall by using HTTP tunneling. Installing Skype btw is one of the easiest things I've ever installed (not counting Linux apps of course, which just install themselves from a repo :-) and that is not a bad feat for a telephony app! (The test call to a echo service is pure genius too.) And it was encrypted of course. So while it is unfortunate Skype isn't Open Source, there is a reason it won.
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