If you have an MX, then you should have and Analog Options Board on the back. It would have both Analog CO ports and Analog Extensions. If you are lucky enough to have an Analog CO line, perhaps a Fax line or Security line, put it on one of the CO ports and program it to be answered by the Voicemail.
Otherwise, it may work to connect one of the Analog extensions to one of the CO ports, again having the CO port answered by Voicemail. Call the Analog Extension from another extension. Not sure if it will work as I have never done it, but if you are in need.....