Why You Press 9 to Buzz Someone In (DTMF Explained)

When someone buzzes your apartment and you pick up the phone, you press 9. The door opens. You've probably done this a hundred times without thinking about it. But what actually happens when you press that key?

What is DTMF?

DTMF stands for Dual-Tone Multi-Frequency. When you press a key on your phone keypad, your phone generates two simultaneous audio tones — one from a low-frequency group and one from a high-frequency group. The combination of these two tones uniquely identifies which key you pressed. This is the same technology that lets you navigate phone menus ("press 1 for billing, press 2 for support").

Your apartment intercom is listening for a specific DTMF tone. When it hears the right one, it sends an electrical signal to an electric strike or magnetic lock on the building entrance, releasing the door for a few seconds.

The frequency pairs behind each key

Each key on the phone keypad produces a unique combination of two frequencies. The low-frequency group runs at 697 Hz, 770 Hz, 852 Hz, and 941 Hz. The high-frequency group runs at 1209 Hz, 1336 Hz, and 1477 Hz. When you press "9," your phone generates 852 Hz and 1477 Hz simultaneously.

Using two frequencies instead of one makes the system resistant to false triggers. A single tone at 852 Hz might occur in background noise or speech, but the combination of 852 Hz and 1477 Hz at the exact right amplitudes and duration is unlikely to happen by accident. This is why your intercom doesn't accidentally open the door when you talk during the call.

Why "9" specifically?

There's no universal standard. The digit "9" is the most common door-release code in North American apartment buildings, but it's far from the only one. Some buildings use "0", "#", "6", or even multi-digit codes like "09" or "11." The code is set by the building management company or the intercom manufacturer during installation.

Common door codes by intercom manufacturer:

• Aiphone: typically "9" (most common residential intercom in North America)
• DoorKing (DKS): often "9" or "0" depending on configuration
• Siedle: varies by installation, commonly "9" or "#"
• Comelit: typically "9" for North American installations
• ButterflyMX: configurable via admin panel, default is usually "9"
• Latch: app-based primarily, but phone fallback typically uses "9"

In-band vs. out-of-band DTMF

On traditional landlines, DTMF tones are sent "in-band" — they travel as literal audio tones through the phone line. The intercom microphone picks up the tones from the audio stream and decodes them. This is simple and reliable because the signal path is direct.

Cell phones and VoIP services handle DTMF differently. Instead of sending audio tones, they often send DTMF as "out-of-band" signals — digital messages embedded in the call signaling protocol (SIP INFO or RTP Event packets). The carrier network then converts these digital signals back into audio tones at the last mile before the intercom receives them.

This conversion usually works fine, but it's where problems can occur. If the carrier's transcoding clips the tone too short, or if a VoIP intermediary (like Google Voice) doesn't relay the DTMF event properly, the intercom may not hear it. This is why some people report that pressing 9 through certain VoIP services sometimes fails to open the door.

Why this matters for automation

Because door unlocking is just a DTMF tone sent over a phone call, any system that can make a phone call and send DTMF tones can open the door. It doesn't have to be a human pressing a key on a phone — it can be software.

This is exactly how BuzzBot works. When your building intercom calls your BuzzBot number, BuzzBot answers the call and runs through a verification flow. If the caller is verified — either as an expected delivery or a recognized household name — BuzzBot sends the DTMF tone programmatically. The intercom receives the tone and opens the door, exactly as if you had pressed the key yourself.

How BuzzBot sends reliable DTMF

BuzzBot uses Twilio's telephony infrastructure to send DTMF tones. Twilio generates the tones server-side and injects them directly into the audio stream of the call. This avoids the in-band/out-of-band conversion issues that can plague consumer VoIP services. The tone is clean, correctly timed, and delivered at the standard amplitude that intercom decoders expect.

If your building uses a non-standard door code (anything other than "9"), you configure it once in BuzzBot's Building Settings. BuzzBot sends whatever DTMF sequence your building requires — "0," "#," "09," or any other combination.

What if my building uses a different code?

BuzzBot defaults to sending DTMF "9" but lets you configure any code in the app. Go to Building Settings and change the door code to whatever your building uses — "0", "#", "6", or a multi-digit sequence. If you're not sure what code your building uses, try pressing different keys next time someone buzzes you, or ask your building manager.

Can DTMF be faked or exploited?

In theory, anyone who calls the intercom and sends the right DTMF tone could open the door. In practice, this isn't a meaningful security risk — the intercom only accepts DTMF tones during an active call session initiated by someone physically at the building entrance. They're already standing at the door. The intercom's job is to let you decide whether to open it; the DTMF tone is just the mechanism for communicating that decision.

DTMF beyond apartment buzzers

DTMF is everywhere in telephony, not just apartment intercoms. Every IVR phone menu ("press 1 for English") uses DTMF. Credit card payment systems read your card number via DTMF tones. Some alarm systems arm and disarm via DTMF codes sent over phone lines. Conference call bridges use DTMF for participant controls (mute, unmute, disconnect).

The technology is from 1963 (Bell System's Touch-Tone service) and has been largely unchanged since. It works, it's universally supported, and it's simple. That simplicity is exactly what makes apartment buzzers automatable — the interface is so basic that a software system can interact with it just as easily as a human can.