I wonder whether the external community have set up their network properly. I sent previously some links to the knowledge center which describe what needs to be done to OCS/Lync servers so that SIP works properly. I would take a look at these again and check whether they have set up reverse DNS entries and SRV records. I'd also check this is the case for you too.
Enabling SIP trace will provide a lot of information. Unless you know what you're looking at you may get a little lost tring to read and follow the SDP headers. If you can, it might be a good time to raise a PMR.