It may be, (but I sort of doubt it), someone else would have to confirm..
Most "add-on" voice-mails systems were/are designed for use with older PBX's, for use in the days before voicemail was integrated in the "package". Systems made by companies such as Octel. They had a number of "lines" run between the PBX and the VM system along with an SMDI (standard message desk interface) link (a low speed 1200/2400/4800 bps) data link. The call information (which mailbox to send a call on which line) was sent to VM system from the PBX, and the VM system would send messages back to the PBX to activate and de-activate the message waiting indicator.
As far as I'm aware, 3CX does not have this capability, as voicemail is incorporated. Other members may be able to provide information on their experiences.