Look at the OBi's Call History. If the international call is present, report how it was handled.
If not, you might have Groups set up such that the calling number won't ring Google Chat -- check the history in GV.
If that's not it, either, try shutting down the OBi, bring up Gmail, have your contact call you again. If Gmail won't ring, it's not an OBi issue and you might try Google support.
If Gmail rings but the OBi sees nothing, check whether anything shows up in syslog, if so, Obihai support may be able to help.
Fonworld is a commercial German VoIP service that offers callback and has a free trial. I haven't used them, but strongly suspect that a callback to your GV would have a +49xxxxxxxxx caller ID, which should be suitable for troubleshooting this problem.