Need help with 24Vdc (2-wire) intercoms

Have a project at work that has my engineers and contractors stumped. So here I turn...

New 8-story office building has a fire alarm system. The building is served by two local Fire Departments. Both departments have radios for all personnel. The new building has a public safety DAS, so both local FD agreed to exclude firefighter phone jacks in the stairs & elevator lobbies. 

The elevators were (mistakenly) installed with firefighter phone jacks in the cabs. Since no good deed goes unpunished.. One local Fire Chief said “remove them, we use radios” and the other said “those need to connect to the fire command room”.  

Electrician installed conduit and pulled wire to make a physical connection from elevator cars (via the elevator controllers) to the fire command room. Now there is a firefighter phone jack on the wall in fire command.

So now you take a red handset off a rack and plug it to a jack on both ends - and you have basically two soup cans and a string. Handsets are the ones shown here.

The problem is those handsets normally terminate into a fire alarm panel that has an phone/intercom card. That card powers the comm circuit with 24Vdc. 

This building FA system doesn’t have an intercom card. So no amplifier.

I’m trying to identify the most economical way to amplify this private intercom circuit so the handsets can be plugged in... and it operates as expected. 

Does anybody here know the answer?

Does the FA panel have the ability to accept an intercom card?

To clarify, the end result you are after is public address capability from the Fire Command Center only?

In reply to paranoid_android :  I need to find out if the panel can accept a card. And yes, that’s the goa - to have intercom from Fire Command room to the elevator cabs. This seems like the obvious answer. I also wondered if there is away to operate this intercom independent of the FA panel. 

Google search 'phone line simulator'.

In reply to TenToeTurbo :

Thanks