From the admin guide (Dial plan and call processing-Page 185 of 479):
1. The VCS sends a query to its DNS server for an SRV record for the domain in the URI. (If more than one DNS server has been configured on the VCS, the query will be sent to all servers at the same time, and all responses will be prioritized by the VCS with only the most relevant SRV record being used.) If available, this SRV record returns information (such as the FQDN and listening port) about either the device itself or the authoritative H.323 gatekeeper for that domain. If the domain part of the URI address was resolved successfully using an H.323 Location SRV record
(that is, for _ h323ls) then the VCS will send an A/AAAA record query for each name record returned.
These will resolve to one or more IP addresses, and the VCS then sends, in priority order, an LRQ for
the full URI to those IP addresses.
If the domain part of the URI address was resolved using an H.323 Call Signaling SRV record (that is,
for _ h323cs) then the VCS will send an A/AAAA record query for each name record returned. These will
resolve to one or more IP addresses, and the VCS then routes the call, in priority order to the IP
addresses returned in those records.
(An exception to this is where the original dial string has a port
specified - for example, email@example.com
:1719 - in which case the address returned is queried
via an LRQ for the full URI address.)
2. If a relevant SRV record cannot be located:
If the Include address record setting for the DNS zone being queried is set to On, the system will fall
back to looking for an A or AAAA record for the domain in the URI. If such a record is found, the call will
be routed to that IP address and the search will terminate. Note that if the A and AAAA records that are
found at this domain are for systems other than those that support SIP or H.323, the VCS will still
forward the call to this zone, and the call will therefore fail. For this reason, you are recommended to use
the default setting of Off.
If the Include address record setting for the DNS zone being queried is set to Off, the VCS will not
query for A and AAAA records and instead will continue with the search, querying the remaining lower