I had a similar problem using a voip service where I couldn't make or receive calls through the obi. went through the obitalk portal, rebooted, changed the settings, changed them back again, deleted the device and added it back in but still it just wouldn't work.
Logging into the obi device itself, I eventually noticed that the firmware was at version 2776 yet all my others were on 2774.
I deleted the obi again, reset it to factory, logged into it again but it was still on 2776 so I set up the unit to a voip service, added it back into obitalk, made some more random changes and it eventually started working again.
I'm now in a position where I can't let it go out into service in case it is actually faulty but I actually suspect that the firmware update was responsible for the malfunction.