Here are the following configurations I applied to a new ASUS AC1900 (RT-AC68U) router that I just bought in order for Wi-Fi Calling to work. Note that I had to apply all three of these settings in order for Wi-Fi Calling to work. Prior to doing this, Wi-Fi calling would not activate on my iPhone 6S even though it would work on other Wi-Fi networks.
1. Enable "Telnet"
Advanced Settings -> Administration -> System -> Enable Telnet -> Yes -> Apply
2. Disable SIP Passthrough
Advanced Settings -> WAN -> NAT Passthrough -> SIP Passthrough -> Disable -> Click Apply
3. Enable Pings under Firewall settings
Advanced Settings -> Firewall -> WAN pings (or Respond Ping Request from WAN) -> Enable -> Click Apply
Hope this helps!
1. Enable "Telnet"
Advanced Settings -> Administration -> System -> Enable Telnet -> Yes -> Apply
2. Disable SIP Passthrough
Advanced Settings -> WAN -> NAT Passthrough -> SIP Passthrough -> Disable -> Click Apply
3. Enable Pings under Firewall settings
Advanced Settings -> Firewall -> WAN pings (or Respond Ping Request from WAN) -> Enable -> Click Apply
Hope this helps!