How Apartment Intercoms Work (and How to Automate Yours)

The apartment intercom is one of the least-changed pieces of technology in urban housing. Most buildings use systems that were installed in the 1990s and work exactly the same way they did then. Understanding how they work helps explain both why they're frustrating and how to fix them.

The basic mechanism

A building intercom is essentially a phone system with a physical panel at the entrance. When someone presses a unit number (or finds a name on the directory), the system calls a phone number registered for that unit.

The resident answers the call. They hear the visitor or delivery person. If they want to let them in, they press a key on their phone — usually "9" — which sends a DTMF (Dual-Tone Multi-Frequency) tone back to the intercom. The intercom interprets that tone as "open the door" and releases the electric strike or magnetic lock on the entrance.

Types of intercom systems

Not every apartment intercom works the same way. There are a few categories, and the type your building has determines what your automation options are.

Phone-based intercoms (most common)

These are the systems described above. A visitor selects a unit, the system dials a phone number, and the resident presses a key to open the door. The phone number can be a landline, cell phone, or VoIP number — the intercom doesn't care. These are the systems BuzzBot works with, and they account for the vast majority of apartment intercoms in North America.

Hardwired in-unit intercoms

Older buildings sometimes have a dedicated intercom handset mounted on the wall inside each apartment. The visitor presses a button at the door, a buzzer sounds inside the unit, and the resident picks up the wall-mounted phone and presses a physical button to unlock the door. These systems don't dial an external phone number — the communication stays within the building's wiring. BuzzBot doesn't work with these because there's no external phone call to intercept.

App-based smart intercoms

Modern systems like ButterflyMX, Latch, and SmartRent use an app instead of (or in addition to) a phone call. Visitors are shown on a video feed, and residents buzz them in through the app. These systems have their own automation features, though most still support a phone number fallback. If your building uses one of these, you may already have some automation built in. For how they stack up against consumer buzzer apps, see the full apartment buzzer app comparison.

What "local area code" means

The intercom stores a phone number for each unit in a database maintained by the building management company. That number can be any working phone number — there's no technical requirement that it be a local area code. However, older intercom systems were designed when long-distance calls cost extra, and some legacy systems are programmed to only dial local numbers. Even modern buildings often have a preference for local area codes because it signals the resident is actually local.

In practice, you can give any number — but a local area code raises no questions.

Common intercom systems

• Aiphone — one of the most common in older buildings; uses a standard phone number
• Siedle — popular in higher-end buildings; phone-based
• DoorKing (DKS) — common in gated communities and larger apartment complexes
• Butterfly MX — modern cloud-based system with resident app; also supports phone number entry
• Latch — app-based with backup phone number option
• Comelit — European-origin, widely installed in US high-rises

BuzzBot works with any system that calls a phone number and accepts DTMF "9" to unlock. That covers nearly all of the above.

The electric strike: how the door actually opens

When the intercom receives the correct DTMF tone, it sends an electrical signal to the door hardware. Most apartment buildings use one of two mechanisms:

• Electric strike — a plate in the door frame that releases when energized, allowing the door to swing open. Common in older buildings. The door re-locks when it closes.
• Magnetic lock (mag lock) — an electromagnet mounted at the top of the door that holds it shut. When the intercom triggers a release, the magnet cuts power briefly, and the door can be pulled open. Common in newer buildings and commercial lobbies.

Both mechanisms give the visitor a window of 3-7 seconds to open the door before it re-locks. This is why timing matters — if a delivery driver is fumbling with packages and doesn't pull the door within that window, they're locked out again.

Why intercoms haven't changed

Intercom systems are infrastructure. Replacing them requires running new wiring, coordinating with building management, getting approval from the HOA or property owner, and paying for installation across every unit. Most buildings don't replace working systems until they break.

The phone-based model also has genuine advantages: it requires no app, no special hardware for residents, and works on any phone. The downside is that it requires a human on the other end to make decisions.

Security considerations

A common concern with intercom automation is security: if a system automatically opens the door, doesn't that make the building less secure? The answer depends on what you're comparing against.

In practice, most residents press 9 for anyone who says "delivery." They don't ask for a tracking number, verify the carrier, or check whether they're actually expecting a package. BuzzBot is actually stricter than the average human — it verifies the carrier against your actual Gmail shipping confirmations before opening the door. If there's no matching email, it doesn't auto-buzz.

For non-delivery visitors, BuzzBot checks the stated name against your household list. Unknown callers get sent to you as a push notification, not auto-buzzed. The automation is selective, not indiscriminate.

How to automate an existing intercom

Because the intercom just calls a phone number and listens for DTMF tones, you can automate it entirely by replacing the human with a programmed system. This is exactly what BuzzBot does:

• You give the building a BuzzBot phone number instead of your cell
• When the intercom calls, BuzzBot answers automatically
• BuzzBot runs through a verification flow: expected delivery → auto-buzz; known name → auto-buzz; uncertain → push notification to you
• DTMF "9" is sent programmatically when the verification passes
• Your real phone only receives the call if BuzzBot can't verify the visitor — and it forwards immediately

The intercom system doesn't know or care that it's talking to software instead of a person. From its perspective, a call was placed and DTMF "9" was received. Door opens.