The simplest way to install a door intercom system is to use a product designed to transmit through your home's wiring. It lets you answer the door from anywhere adjacent to an electrical socket at your home. You will be able to talk to people outside your door without actually opening it, and you can use much of the installation from your previous doorbell.
Flip the circuit breaker that supplies power to your existing doorbell transformer. Check the power is off by pressing the doorbell and confirming it doesn't sound the bell.
Disconnect and remove the existing doorbell button.
Take the door intercom bracket and mount it to the wall using the screws provided. Mount it so that it covers the mark and any wires left by the old doorbell if possible.
Connect the existing wires to the contacts on the door intercom module. Don't worry about which wire goes to which terminal; any will do. Attach the door intercom to the bracket.
Disconnect the existing wires from the existing transformer. It's inside the home. Disconnect the existing wires from the existing doorbell which may be nearby.
Route the 18 AWG bell wire from a nearby power outlet to the location of the old transformer. Don't poke any wires in the power outlet.
Connect the new wire to the wire at the old transformer using wire nuts. Connect the other end of the new wire to the terminals on the new power adaptor and plug it in. Turn the circuit back on by flipping the power again.
Plug the intercom into the power supply nearby where you want the intercom located.
Press the new doorbell button and the bell will sound. Anyone at the intercom can now press the "Talk" button and speak to the person at the door. The person at the door can press their "Talk" button to converse.