Alternate Host / Failover
Digium phones, beginning with phone firmware 1.1 and DPMA 1.3, support simultaneous registration to two hosts. In the event that the primary host is unreachable, calls are be placed to the secondary host instead. When the primary host is down, only call services are supported on the alternate system; status, voicemail, parking, queues, etc. are not functional. Registration to the alternate host is done over normal SIP registration, not via DPMA, so no res_digium_phone entry for the phone is required, only PJSIP (pjsip.conf) configuration.
When the primary host is unavailable, the phone will display a failover icon in 20-30 seconds. When the primary host becomes available again, the phone will switch back to it once the phone detects its availability. In DPMA, registration to this alternate host is controlled via the alternate_registration_address and alternate_registration_port network options. If those options are present, the phone will apply that alternate host to all internal lines. For phones manually provisioned via XML, see the host_alternate XML configuration parameter.