Between Head office and Branch we have two VPN tunnels: main and backup. As IGP we are using OSPF. When I shut down main VPN tunnel, OSPF installs routes through backup VPN which takes aproximately 2-3 seconds. During that time IP Phones (Alcatel-Lucent) are restarting and we lose phone connection ~1minute.
So what to do, that IP Phones stay connected?
You will need to investigate if you can adjust the ip phone keep-a-live
time between your IP phone server & the ip phones.
Sounds like the timer is set at around 3 seconds if you could increase that to 10 seconds
that will give ospf time to converge.
Hope this helps