But the point is that even though the dream of the SIP URL, which was intended to become like an E-mail address for VoIP, did not get realized, the number of people you could call on SIP phones was still too low for Skype to bother allowing it. At the time I did my study a year ago, there were perhaps 5,000 logged on to FWD at any given time, a few thousands more in the other SIP networks. Vonage allowed SIP interconnect for a while but shut it down. (No, calling somebody with a PSTN termination is not calling somebody with SIP, and in any event, Skype did that pretty quickly.)

If you could find their URLs, there are a modest number of companies and institutions that let you call their phone system with SIP, but the number then, and still today, was a few hundred thousands. Skype now reports typically over 3 million actually logged on and ringable, and far more installed.

I mean to have another thread about this, but ask yourself, how many SIP phones can you ring right now with the normal UI of your device? SIP is being used primarily for PoIP right now (PSTN over IP)