Is it possible to change the DNS server for WAN 2 primary to 220.127.116.11 and secondary to 18.104.22.168 ?
If the configuration is dynamic, there should be a check box to use different DNS servers. Let's give this a try to see if the phone does not care for the DNS configuration.
What is the firmware version on the router, also, can you reboot the router?
Firmware is v4.2.1.02 (Jan 18 2012 14:10:55)
I already checked for more recent firmware. My version seems to be the most current.
I will reboot the router and report the result here as soon as it is up again.
Thomas, are you still there?
I have to admit I changed the DNS settings back to "automatic" out of "aestetical reasons", and because I didn't understand why DNS plays a role (we are dealing with IP addresses, not domain names).
After "Registration failed" I switched back to 22.214.171.124/126.96.36.199 and rebooted. First again "Registered", then after some minutes "Registration failed".
It looks like if I solved my problem. The key hint was Thomas' remark that in the event of WAN failure, protocol bindings fail over to the working WAN. In my setup I had Network Service Detection enabled with Default Gateway selected as the target for the ping attempts. What I was observing in the System Log were strange messages of "NSD FAIL WAN". I did not understand these because WAN1 was working fine. Now I had the idea to try to ping the Default Gateway of WAN1 manually in a command prompt window and obtained "Request timed out". I guess this machine simply drops ICMP packets but works OK as a gateway since I do not experience any problems using WAN1 for internet access.
Now I disabled Network Service Detection and since then my IP-Phone keeps being registered via WAN1 with my DSL provider.