I would buy a cheap appliance like a Grandstream UCM … can get one for around $200. Get some cheap IP phones to register to it and you can do intercom calls at no cost. You can add your analog line or a SIP trunk to the system to allow all of those phones to answer the main line or make an outbound call (which you should probably have in that environment for emergency purposes).
“How to start a telco in 3 easy steps ….”