Not too long ago our building replaced its intercom with a system that connects the front door microphone and speaker to a local phone number. The old system was broken and this was the replacement. Why replace it at all? Because New York State has a law requiring intercoms in many residential buildings:
Every class A multiple dwelling erected or converted after January first, nineteen hundred sixty-eight containing eight or more apartments shall also be equipped with an intercommunication system. Such intercommunication system shall be located at an automatic self-locking door giving public access to the main entrance hall or lobby of said multiple dwelling and shall consist of a device or devices for voice communication between the occupant of each apartment and a person outside said door to the main entrance hall or lobby and to permit such apartment occupant to release the locking mechanism of said door from the apartment. (NY Mult Dwell L § 50-A (2012))
But for a building to use a phone-based system defeats the purpose of the law — if the phone is in use or off the hook, the person buzzing from downstairs is simply connected to voicemail. We now pay extra money for call-waiting, but even so visitors sometimes tell us they have been connected to voicemail when buzzing from downstairs, because involving the phone company has increased the number of things that can go wrong with the system.
Anyway, unless you live alone, a landline is a necessity while the building is allowed to have a phone-based intercom-buzzer in order to fulfill section 50-A of the Multiple Dwelling law.