It comes down to however many phones you need. I installed a Panasonic 616 in a factory about 20 years ago. They actually could page overhead and receive calls to the phone. Pana's pickup dial gave the interface to the paging after a 2 second delay. If they dialed fast enough, they could ring an extension.

This type of arrangement would probably be the least expensive. You have $100 or so for an 8-port key system, and the interface is 2 wires to each phone, and 2 wires to the paging system amp. 16 extensions might be about $175 and maybe another $100 for parts. Ebay would be your friend.

The MIC arrangement would tend to be cumbersome as you would have to map out and balance each connection and distance to the amp. Sticking to a telephone based system is more forgiving.

Carl