There's a good chance this is due to Whistlephone returning a non-standard SIP result code. If the OBi110 receives a non-standard SIP result code, instead of simply treating it as an error, it reports that the service provider didn't respond even though it did. I've been meaning to report this to support but haven't gotten around to it.
I have a Whistlephone account and will try it a little later to see what my OBi does with it. It should work for outgoing calls, but will likely never work for incoming calls as Whistlephone appears to use 5 different servers and has to be registered to all them to have much of a chance of receiving a call (this is according to Asterisk folks).