There are several DNS based registries (zones) where someone using VOIP can publish their SIP (or other) connection information. When a device like an IPPBX needs to connect to another internet phone or IPPBX, it can query DNS to find the service information.
For example, if you have a phone number you can query DNS in the e164.org, or e164.arpa namespaces (there are others) for DNS NAPTR records that describe how to connect to the machine handling the phone number. If the number has been registered in DNS, you will get the service type and a regular expression (regex) defining how the connection is to be made. The regex will have a portion delimited by the exclamation point character called a URI, typically in the format !sip:firstname.lastname@example.org! -- this part is used along with the service type to make the connection to the machine receiving the phone call via SIP or H323.
Our freeware ENUMresolver make it easy to see if the records are in DNS and it displays their format. It accepts a telephone number and can query your default DNS or a specific DNS for the records and displays them. You must include the country code in front of the phone number. Numbers in the North American Numbering Plan (US/Canada/Carribean) need to include the number "1" in front of the phone: 1 (360) 999-9999 is acceptable US format in ENUMresolver.
We just released v2.00 on November 19, so you can now tell it to query a specific DNS and it also has a list of root zones for you to select from -- of course you can enter any root zone you want.
You can get it at our freeware page.