SIP2SIP

One Contact number (oneworldonecontact.com) is a universal 12 digit phone number to reach VOIP users across all SIP providers. Way it works is, whenever any user dials a 12 digit number (from SIP phone dialer) SIP proxy, used by the user, checks for the registered 12 digit user id and process the calls normally. If there is no user registered corresponding to the number dialed, SIP proxy makes call to itspexchange.org (http://www.itspexchange.org/resolve.php?oC=<12-digit number>) to get Proxy for the number. SIP proxy of the original caller then forwards the request to the proxy obtained from itspexchange.org.

12 digit One Contact number is owned by the users (not SIP Service providers), so users have flexibility to change SIP Service providers without loosing their contact number. One Contact numbers, in bulk, are initially allocated to SIP Service providers who in turn assign them to their customers. Once a One Contact number is assigned to a customer (individual/company/organization) they become the owner of the number (similar to the concept of cell phone number portability). SIP Service providers provide updates on the users assigned the number to oneworldonecontact.com. (oneworldonecontact.com can be contacted to get the numbers.)

I know that utility of the 12 digit One Contact number greatly depends on the number of SIP service providers supporting it, but big SIP Service providers such as SIP2SIP can still benefit from it as there are large number of current users who could interact with each other using the 12 digit One Contact Number.