Building a Community VoIP Directory Server?

Christopher Rath asks: "Earlier this month, a reference to Clay Shirky's piece on ZapMail was posted as a Slashdot article. An obvious (to me) next step, which was hinted at in a couple of the postings which followed, was for the Internet community---you and me---to put up a VoIP directory server. I want to use an Analog Telephone Adapter (ATA), like the Cisco ATA or The Kompany's new tkcPhone, to talk to other guys who have ATAs too; I have very little interest in making a local call using the ATA (for that I've still got my Bell land-line). Surely someone has already started such a project, if not, why not?"

"To make investing in an ATA worthwhile, I need an easy method of establishing a connection. In the current broadband environment, where most of us don't have static IP addresses, this means that we need our ATA to register itself with some VoIP directory server that can be used to assist with "dialing the number": as a user I want to remember my friend's ATA number, and then the directory server maps this to the ATA's current address. Once my ATA has the other ATA's address, the call should be ATA-to-ATA without any burden on the directory server."