Joining to a Sip Federation allows you to use a distinctive ringing for distinguish in between an authenticated caller or not. This should help in screening of undesired, anonymous calls, without the need of maintenance of an explicit buddy list of white-listed contacts.
Distinctive ringing happens if your User Agent correctly reacts to the presence of a special Auth-Info header in the SIP message it receives. This header is inserted by the caller's SIP proxy after digest authentication of the caller, and preserved by intermediate Proxies if they can authenticate each other.
Inter-entity authentication is possible because of the sharing of a common Certification Authority, which is publicly announced by the insertion of a well-crafted NAPTR Resource Record in the authoritative DNS zone of each entity.
Who can join, how, and why
As a network cant be called as such, until many nodes take part of it, and as new approaches aren't widely adopted until they become common practice, we realized that the main efforts must to be spent toward the maximum applicability of the technique. For this, we developed an architecture which is open to contributions coming from either static or dynamic IP hosts. You just need a Linux Box (we use Ubuntu for its extreme ease of use), and an Internet connection giving a public IP to your router/NAT. Then, you can start to offer an authenticated VoIP service to your family and your friends.
You could now say.. why don't simply use Skype? Oh, very easy.. because SIP is an Open Standard!
Joining to our SIP federation is not so difficult, still not so direct. The main passages can be resumed as