If the default fall-back delay is not acceptable for your architecture you can use the following advisory documents from Polycom:

http://supportdocs.polycom.com/PolycomService/support/global/documents/support/technical/products/voice/SIP_Server_Fallback_TB5844.pdf
http://supportdocs.polycom.com/PolycomService/support/global/documents/support/technical/products/voice/DNS_Cache_TB36033.pdf
http://supportdocs.polycom.com/PolycomService/support/global/documents/support/technical/products/voice/Configuring_Optional.pdf

http://plcmtechnet.com/documents/voice-conferencing/unified-communications-software-ucs/5-1-0/administrator-guide/set-advanced-phone-features#_Configuring_the_Static

 

Below you can find an example of this recommendations applied on a live environment:

Components

  1. Servers -03

  2. Ethernet Switch -01

  3. IP phone with DNSSRV functionality -02

  4. FXS Gateway with DNSSRV Functionality -01

Setup Diagram & Configuration-


 


Step 1- Enable the Unmanaged Service in DNS Option as shown below

 

As an alternative in later versions of Sipxcom you could use custom records to edit a new entry in DNS configuration

Or you can simply use an external BIND server.

Step 2 - Edit priority according to below diagram


Step 3 - Set TTL parameter -same value for both servers



 

 

 

Note:

Special thanks to Amit Kansal and Rakesh Panwar who tested and implemented this scenario