So you have run into the reason that Exchange UM isnt supported for CUCM 4.2.3. Inbound messages are routed to to 5060 and depending on what process you require can hit either 5065 or 5067. I think the UM worker process is 5065 from memory. Best bet is either move to a supported version of CUCM or place an Cisco ISR configured as IP-IP GW(SBC) between the two deployments. I have posted about this configuration at my blog. You could use a already deployed ISR to complete the job.