You should check the call history and see what it says. You can also do a test call and check the call status during the call.
To access the above you need to go into the device itself using the device's ip address. You can obtain the ip by dialing ***1 from the phone port.
With call forward set at 4 rings it is possible that google voicemail is taking the call before the Obi forwards it. Google voicemail picks up after 25 seconds from original call start, no changes can be made to that setting.
Try 3 rings and see what happens.