The easiest way to get it would be from any windows (2000 or xp) machine that you can use to ping the medpro card. Your PC and the cards you are checking on will have to be on the SAME SUBNET for this to work. (And, of course, you would have to know their IP address.)
Open a command line window on your PC.
Then, ping the medpro card:
ping xxx.xxx.xxx.xxx
Finally, use the arp command to read the arp table in your pc:
arp -a xxx.xxx.xxx.xxx
Pinging the voicemail machine will make your windows computer have to talk to the medpro card. Windows makes a temporary entry in the ARP table of your pc with the IP address and the MAC address as a result. The windows arp command lets you read what it is.
Carpe dialem! (Seize the line!)