since the outbound calls go through your PaBX, it would be good to check the call manager traces for the calls.
check the traces for the call-leg to the gateway, 2801, and the call-leg out of the gateway to the PaBX.
there should be clear indication of the call status in the traces and which component, gateway or beyond, is invoking the disconnect.
it does seem clear that the problem will be at the PaBX but the traces can confirm. it seems the PaBX is not sending the proper connect codes, etc. to call manager.
see this link for more call manager trace info:
http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_administration_guide_chapter09186a00801ed111.html