@nitinmits You will not be able to ping the VOIP gateway over the wifi or from any other connected PC. You will need to test it from the router diagnose page and select the VOIP WAN (I have it set up as bridge so I can't show you a screenshot from the ONT UI)
In my case, I can very much ping the gateway and the SIP server from my OpenWRT router where the VOIP interface is configured.
@Nikhil Sharma Could you please show the screenshots of the following pages
WAN status page (where it shows the connection status of the internet WAN etc)