You are saying it was working but now doesn not?
Any network changes recently? Would think that if the audio gets to the desk phones fine then it is not likely a 3300 issue ( or even a UCA ). When the audio of a DID call hits the 3300 it gets streamed to the desk phones fine. I assume the desk phones are on the same voice VLAN as the 3300 so that is why they work. If the DID audio hits the 3300 and it is to be answered by a UCA softphone setup to take the call, then the audio from the 3300 needs to get to the IP of the computer the UCA softphone is on. This is likely not the voice VLAN rather it will be the data VLAN. The data VLAN info would need to be in the local networks programming of the MAS with the UCA and there would need to be network routing to allow voice packets to reoute form the voice VLAN to the data VLAN.