When making a test call to **9 222 222 2222 or making a Google Voice call, my Obihai seemed to use a DSCP value of 0x2E (decimal 46). This seems to be EF (Expedited Forwarding). I also noticed that in the Obihai device interface under Service Providers, ITSP Profile, RTP, DSCPMark = 46.
Google doesn't give destination server IP addresses or names (according to
https://support.google.com/a/answer/1279090?hl=en), but as an alternative, you could give your Obihai a static IP address on your local network and then configure your router to match the Obihai's IP address. Another alternative would be to have your router match the DSCP value, since that might also help other devices or apps such as Google Hangouts on WiFi devices.