The 2X12 is not capable of knowing the line is in use outside of itself. Not even if it's another 2X12. One of my ongoing frustrations with them. For Plain Ol' Analog Copper Telco Lines at least, the older Telos 1X6 is a much better option, especially if you're sharing lines. However! If an external phone picks up a line, the 1X6 will not be able to pick it up when the external phone puts the line on hold. It will continue to show the line is in use and that locks out offhook on that line. Fought that feature trying to get a game broadcast on the air once. But if you have call transfer, you could "transfer" the call to another line if the studio knows it's supposed to pick it up before the other operator picks up the phone!
Seems like some kind of external indicator is called for. (I'm thinking beehive lamps. Remember those? Two sets of three lamps to show the lines which are in use or on hold and one white one to show one of them is ringing the studio. Don't know what you'd drive that with today, except for the white one with ringing voltage, absent a 1A2 key system! Enough wishful thinking.)
What kind of phones are the volunteers using? Do they even have "hold" buttons? A camera might not be a bad idea. Could a 7th volunteer be stationed at a 2nd 2X12 instrument set to Producer mode and pick up lines being handed off and put them on "screened hold?" Every possibility just seems clunky. Has to be an elegant solution.
Salem Media Group Omaha