Yealink SIP-T2XP Administrator's Manual page 186

Administrator's Guide for SIP-T2xP and SIP-T19P IP Phones
Fallback: In this mode, a second less featured call server with SIP capability takes

over call control to provide basic calling capability, but without some advanced
features offered by the working server (for example, shared line, call recording
and MWI). IP phones support configuration of two SIP servers per SIP registration
for fallback purpose.
Phone Configuration for Redundancy Implementation
To assist in explaining the redundancy behavior, an illustrative example of how an IP
phone may be configured is shown as below. In the example, server redundancy for
fallback and failover purposes is deployed. Two separate SIP servers (a working server
and a fallback server) are configured for per line registration.
Working Server: Server 1 is configured with the domain name of the working server. For
example, yealink.pbx.com. DNS mechanism is used such that the working server is
resolved to multiple SIP servers for failover purpose. The working server is deployed in
redundant pairs, designated as primary and secondary servers. The primary server has
the highest priority server in a cluster of servers resolved by the DNS server. The
secondary server backs up a primary server when the primary server fails and offers
the same functionality as the primary server.
Fallback Server: Server 2 is configured with the IP address of the fallback server. For
example, 192.168.1.15. A fallback server offers less functionality than the working server.
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