From FreeSWITCH Wiki

DNS

Here we list several types of DNS records that integrate quite nicely with FreeSwitch and IP communications. 

ENUM

A way to query e.164 numbers to keep calls "on-net".

SRV

These records can give you a pseudo redundant network. Allowing failover (or pseudo load-balancing) to other servers in your network.

ITAD (ISN)

E-Mail style phone numbers that more easily allow cross-domain calls to stay "on-net" without the need for a full keyboard on your telephone (i.e. analog telephones hooked up to analog adapters). A typical number will look similar to 123*543.

Updating Records

There are many possible ways to dynamically update records in running DNS zones. Here we list the ones we know.

Generating A DNS Key

dnssec-keygen -a HMAC-MD5 -b 512 -n USER <NAME_OF_KEY_HERE>

nsupdate

All of the following commands are input from the nsupdate command line. You should first have a key that you can use for updating a single zone.

ENUM

update add 3.2.1.enum.org 60 NAPTR 100 10 "u" "E2U+sip" "!^.*$!sip:123@freeswitch.org!" .

If a user dials 123, this will route the call to 123@freeswitch.org provided enum is correctly configured to use your zone.

ISN

update add 3.2.1.543.freenum.org 60 NAPTR 100 10 "u" "E2U+sip" "!^.*$!sip:123@freeswitch.org!" .

This would route 123*543 to 123@freeswitch.org

SRV

update add sip.example.com 60 SRV 100 10 5060 sip1.example.com
update add sip.example.com 60 SRV 100 10 5060 sip2.example.com

The preceding will allow load-balancing between sip1.example.com and sip2.example.com

update add sip.example.com 60 SRV 100 10 5060 sip1.example.com
update add sip.example.com 60 SRV 100 100 5060 sip2.example.com

This will keep sip2 on stand-by for use if sip1 fails. This works for User-Agents that properly support SRV.