I loved John Todd's ETel presentation (podcast) on FreeNum, a scheme for bringing phone numbers to the Internet. Of course, I love identifiers and addresses and all that they enable, so it was a natural.
Suppose you were a university campus and when you looked at your phone bill, you noticed that a lot of calls were to other universities. You've got a VoIP telephone system; they've all got VoIP telephone systems. You might wonder "isn't there some way to route these calls over the Internet and save some serious money?"
The answer, of course is "yes" but making it usable is a little harder than simply routing packets. The problem is all about identifiers and addresses. In this case, identifiers that are addresses.
Traditional phone numbers are tightly controlled by the telcos, unlike the world of Internet addresses. The simple answer would be to use something like SIP URLs, but we run into a usability problem: most people's phones have a regular, standard-issue DTFM 12-key dial pad. Using that to enter SIP URLs is a non-starter for anyone but the most hard-core.
ENUM could provide some help. ENUM is method for providing DNS-like services for phone numbers that piggybacks on DNS. The problem is that it's not very DNS-like. In DNS, control of a subdomain can be delegated (the zone file), but with ENUM, it's hard to delegate responsibility for zones to the right entity. This is because ENUM is based on traditional telephone numbers and so the zones don't necessarily match with entities who care. For example, who should responsible for the "3" zone inside the 801 area code?
Todd discusses an alternative approach called ISNs. ISNs rely on a new number, administered by IANA called an ITAD or Internet Telephony Administrative Domain. Like a domain, anyone can get an ITAD. Once you have one, you control the naming inside that number, just like you control the email addresses inside your domain.
ITADs are combined with an internally assigned number (called the subscriber number) to create an ISN. So, suppose that BYU's ITAD was "256" (it's not). My extension is "26465" so my ISN would be "26465*256". Someone at another entity with a SIP phone could call me, from a regular keypad, by calling that number.
Update: I wasn't far off. BYU's ITAD is 458. I don't know that 26465*458 works as my ISN, however.
It seems that my i-name registrar ought to be able to apply for an ITAD and then assign a number to be that resolves to by i-name and then use the information in my XRDS file to route the call for me--even to my cell phone if that's where I've pointed xri://=windley(+phone). That's a service I'd like to see.