The problem
The landline died of spam. Not of mobiles — of the fact that picking it up became a gamble, and a house full of people learned to let it ring. A phone nobody answers is a phone nobody calls, and a phone nobody calls is the one a child cannot use to reach a friend.
What the lobby does
A call arrives. If the number is on the house’s list, the caller hears “Welcome, I’ll connect you” and the house rings — every handset, or the rooms a ringer ladder names first.
Everyone else hears “Welcome to the phone lobby. Please dial an extension.” A friend with the six digits for a room dials them and that room rings. A caller with nothing to dial, or the wrong thing, hears “Good day.” and a click.
That is the whole mechanism. There is no spam list to maintain and no guessing at which area codes are safe. The doorman does not decide who is a spammer; it decides who is expected, and treats the rest with courtesy and brevity.
Why it holds up
Extensions are passwords. A six-digit extension exposed to the whole phone
network is a million combinations, which is plenty against a person and not
much against a machine that redials. So after three failed calls in an hour a
number skips the greeting and goes straight to “Good day”: no prompt, no dial
window, nothing left to guess at. The extensions themselves are generated at
random, never chosen, and doorman rotate replaces them in a second.
Ten seconds is for the first digit, not all six. Somebody reading a number off a card gets ten seconds to start and three between digits, so a confident friend is through in two seconds and a fumbling one still gets a fair go.
Nothing a stranger types is written down. A wrong extension is logged as wrong, never as what it was: a near miss is most of a credential.
Numbers are tidied before they are compared. Carriers hand over the same
caller as 5125550100, 15125550100 and +15125550100 on different days.
Everything folds to one form first, so Grandma does not meet the doorman a
third of the time.
Details worth knowing
Caller ID is presentation, not proof. A known caller skips the lobby on the strength of a number anyone with the right equipment can assert. That is why the list decides who skips a greeting and nothing more; anything that acts on a caller’s word needs a second factor, and gets one.
The lobby screens incoming calls only. It is not an approved-contacts dialler. Outside a bedtime, the supplied dialplan lets a handset call ordinary US and Canadian numbers.
Withheld caller ID always meets the doorman, and all anonymous callers share one failure budget. One persistent blocked caller can dismiss the others faster. That is the intended trade.
The voice is yours to choose. The doorman’s lines are a folder of audio. Swap in another pack and a Victorian doorman, a bureaucrat or a ship’s computer meets your strangers instead.