You are correct about OHVA on older phones. They actually had an extra PC Data Port Module you installed to break out the second audio channel (they used something similar to ISDN BRI...2 B + D. The second channel could be used for OHVA, you could actually have a two-way conversation on speaker while user had live call on handset. Or you could use the second channel to break out a second analog extension right at the phone using MDPM (modem data port module).
But as lowradiation said, sounds like all you need is standard HANDS FREE intercom.