Three short steps. Nothing to install on a computer, nothing to type on your phone.
This is what visitors see on the screen and on the door sign.
On the phone you will answer from, open the camera and point it at this code. It opens the desk, already connected to this screen. Then put the desk on that phone's Home Screen — the desk shows the steps for that phone, because an iPhone never offers it on its own. For alerts that always arrive, also turn on Get alerts on your phone on the desk.
Answering from a computer instead?
Open it on that computer. Keep it private — anyone with this link can answer as your front desk.
You can pair more phones later from Settings.
So it opens full-screen, with nothing else to tap. Do this on the tablet that will sit in the lobby.
No screen at the door? Print the door sign instead — visitors scan it with their own phone. Wording, photos of the rooms and payments live in Settings.