Failover redundancy can only be utilized when the configured domain name of the SIP
server is resolved to multiple IP addresses. If the IP phone is not configured with a DNS
server, or the DNS query returns no result from a DNS server, you can configure a set of
DNS NAPTR/SRV/A records into the IP phone. The IP phone will attempt to resolve the
domain name of the SIP server with static DNS cache.
When the IP phone is configured with a DNS server, the IP phone will behave as follows
to resolve domain name of the SIP server:
The IP phone performs a DNS query to resolve the domain name from the DNS
server.
If the DNS query returns no results for the domain name, or the returned record
cannot be contacted, the values in the static DNS cache (if configured) are used
when their configured time intervals are not elapsed.
If the configured time interval is elapsed, the IP phone will attempt to perform a
DNS query again.
If the DNS query returns a result, the IP phone will use the returned record and
ignore the statically configured cache values.
When the IP phone is not configured with a DNS server, it will behave as follow:
The IP phone attempts to resolve the domain name within the static DNS cache.
The IP phone will always use the results returned from the static DNS cache.
IP phones can be configured to use static DNS cache preferentially. Static DNS cache is
configurable on a per-line basis.
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