Limitations of the phone when the phone is in the process of failover or failback are as follows:
Held calls are dropped.
Calls that are in the middle of the conferencing or transfer set up are dropped.
Calls in the dialing or ringing state might not be completed.
Emergency calls might not work depending on the stage of failover and the functionality available on the alternate server.
Incoming calls might not be completed, or they might get diverted to voicemail.
Message Waiting Indicator is cleared.
