If the call is set up ok and the problem is one way speech, then the problem is likely to be the RTP ports being used. RTP is used to transport speech.
Try putting the OBi100 into DMZ of your router. If this works, then take it out of DMZ and port forward in your router the relevant RTP ports. In your OBi100 look under Service Providers > ITSP Profile A (for sp1) > RTP to see which ports are used for RTP.