Recently I decided to take a look at E164.org; a directory of telephone numbers that can be reached over the Internet.
I have an Asterisk PBX installed in my home+office with incoming service through ISDN (provided by the Telco; Telstra) as well as interconnects with various VoIP providers.
E164.org allows people with telephone numbers also reachable via the Internet (eg: VoIP) to publish them in a centralised database. Oher E164 applications including Asterisk can then lookup details when finding the best path to direct a call.
For example: let’s say somebody with an Asterisk PBX configured for E164.org lookups decides to call one of my phone numbers. Instead of routing the call over the (potentially expensive) PSTN, the call can be sent straight over the Internet to my Asterisk PBX.
Of course, if either side’s Internet connection is down, the call can simply proceed as normal over the PSTN and the caller wouldn’t know the difference.
The number of destinations accessible via E164.org is limited at the moment, but hopefully it will grow as time goes by.