How do internal phones locate the PBX after failover

The answer depends on the location of your two PBX’s. If the two PBX’s are located on the same subnet, then

1. Move IP: Use the VoIPNIC option of HAAst to move a single IP between peers. This will allow for rapid reconnection of downstream (user agents) and upstream (trunks)

If the two PBX’s are located on different subnets (from each other):
[list=2]
[*]SRV records: Assuming your user agents (phone sets) support SRV records (which most do), then you should create SRV records for your two PBX’s. Most user agents will perform a DNS lookup for SRV records to find available PBX’s, and try them in order of priority until they successfully register with a PBX. For example, if you have PBX’s located in data centers dc1 and dc2, create two DNS entries (in your internal DNS server) as follows:

type=srv
name=_sip._udp.mydomain.com
priority= 10
weight=0
port=5060
hostname=pbx1.local

and

type=srv
name=_sip._udp.mydomain.com
priority= 20
weight=0
port=5060
hostname=pbx2.local

[*]Route Change: Use the pre/post Asterisk start/stop event handlers of HAAst to update routes in your router(s). Set the updated routes to point to the new PBX address.
[*]DNS update: Use the pre/post asterisk start/stop event handlers to update a public DNS entry. Be sure to set the TTL value low enough that phones will lookup the new IP in a reasonable timeframe.[/list]

Note: Using SRV records or DNS entries makes it easy for users with softphones to move on and off LAN and resume a PBX connection without manual intervention.