Yealink SIP-T2 SERIES Administrator's Manual page 687

"_sip._tcp.yealink.pbx.com". If the flag of the NAPTR record returned is empty, the IP phone will
perform NAPTR query again according to the previous NAPTR query result.
SRV (Service Location Record)
The IP phone performs an SRV query on the record returned from the NAPTR for the host name
and the port number. Example of SRV records:
Priority Weight
IN SRV 0 1
IN SRV 0 2
Parameters are explained in the following table:
Parameter
Specify preferential treatment for the specific host entry. Lower
Priority
priority is more preferred.
When priorities are equal, weight is used to differentiate the
Weight preference. The preference is from highest to lowest. Keep the same
to load balance.
Port Identify the port number to be used.
Target Identify the actual host for an A query.
SRV query returns two records. The two SRV records point to different hosts and have the same
priority 0. The weight of the second record is higher than the first one, so the second record will
be picked first. The two records also contain a port "5060", the IP phone uses this port. If the
Target is not a numeric IP address, the IP phone performs an A query. So in this case, the IP
phone uses "server1.yealink.pbx.com" and "server2.yealink.pbx.com" for the A query.
A (Host IP Address)
The IP phone performs an A query for the IP address of each target host name. Example of A
records:
Server1.yealink.pbx.com IN A
Server2.yealink.pbx.com IN A
The IP phone picks the IP address "192.168.1.14" first.
Outgoing Call When the Working Server Connection Fails
When a user initiates a call, the IP phone will go through the following steps to connect the call:
Sends the INVITE request to the primary server.
1.
If the primary server does not respond correctly to the INVITE, then tries to make the call
2.
using the secondary server.
If the secondary server is also unavailable, the IP phone will try the fallback server until it
3.
Port Target
5060 server1.yealink.pbx.com
5060 server2.yealink.pbx.com
Description
192.168.1.13
192.168.1.14
Configuring Advanced Features
663