ENUM
For a general introduction to ENUM, see here.
The easiest way to get your own ENUM number (a mapping from your existing E.164 (PSTN) number to your existing SIP URI) is to sign up for a free account with e164.org. (Site not accessible)
SIPbroker's "ENUM with PSTN fallback" service (and the *013 ENUM prefix) currently checks only e164.arpa and e164.org. The query part is single-threaded, but once it is rewritten in multithreaded form, we would like to also check any other helpful ENUM roots, perhaps including DUNDI, PhoneGnome, FWD, and VPF.