What Pressing 9 Does on an Apartment Buzzer

What actually happens when you press 9?

Your buzzer call is a normal phone call. The lobby panel dialled a number, and you picked up. When you press 9, your phone does not send a message anywhere. It plays a sound into the call.

That sound is two tones at once. The 9 key plays 852 Hz and 1477 Hz together. The pair is called a DTMF tone, short for dual-tone multi-frequency, and the frequencies are standardised internationally in ITU-T Recommendation Q.23.

Every key uses one frequency from a low group of four and one from a high group of four, so each key has its own pair. Q.23 allows each frequency to drift only 1.8% from its nominal value. A panel listening for 9 is listening for that exact pair.

The panel in your lobby listens to the call audio the whole time you are talking. When it hears the pair it was programmed to expect, it closes a relay. The relay releases the door lock. On a Mircom TX3 the door stays unlocked for 10 seconds by default, and the building can set anywhere from 1 to 60 seconds.

1. The visitor finds your unit Your visitor presses your unit code on the lobby panel. The panel looks up the one phone number stored for your apartment.
2. The panel places a call The panel dials that number over a phone line. It is a real outbound phone call, not an app notification.
3. You answer Your phone rings and you talk to the person at the door. A Mircom TX3 cuts the conversation off after 60 seconds by default.
4. You press the digit You press 9. Your phone plays the two tones into the call, the same way it would if you were navigating a menu.
5. The panel decodes it The panel picks the tones out of the audio, checks them against its door-open setting, and closes the door relay.
6. The door unlocks The lock releases for the programmed time, 10 seconds by default on a Mircom TX3. On a current Enterphone the visitor hears please enter, the door is unlocked.

Why did your building choose 9?

The door digit is a setting inside each panel, not part of any telephone standard. Q.23 defines the tones, not what a panel does with them. Nine is familiar because some panel makers ship with it.

A Mircom TX3 has a setting called Main Door DTMF. The factory value is 9. The building can set it to any digit from 1 to 9, or to * or #.

DoorKing panels get there by a different route. On the 1833, 1835 and 1837, relay 1 ships with a four-digit tone open code of 9876. The first digit is the momentary open, so pressing 9 releases the lock for the strike time. The other three digits hold the relay open, release it, and hold it for an hour, functions DoorKing describes for management.

So on a Mircom or a DoorKing left at factory settings, a resident who presses 9 opens the door, for two unrelated reasons. A current Enterphone, made in BC, ships with 6 instead.

Is 9 the same digit in every building?

No. Treat 9 as a good first guess and nothing stronger. Four things move it.

• Second doors get their own digit. On a Mircom TX3 the auxiliary door defaults to 6, for a door such as a garage or a side entrance.
• Some digits are reserved. Mircom uses 4 to refuse entry and drop the call. On some Mircom controller boards, 1, 7 and * are off limits too.
• Some buildings set a per-unit code. A Mircom TX3 lets management give one apartment its own security code of up to four digits, entered then #. That replaces the building digit for that unit only.
• Makers differ. A current Enterphone ships with 6 for the main door and 8 for a second door, so 9 does nothing there unless the building changed it.

The reliable answer is on the sheet your building handed you at move-in, or from the property manager. If your building runs a Mircom, note that changing the number the panel calls is a separate job from changing the digit. If the digit does not work at all, the cause is often the call rather than the digit, and press 9 does not open the door covers that case properly.

Why is the tone the fragile link in the chain?

Everything before the digit is tough. The panel dials, your phone rings, and you talk. Voice survives a lot of damage and still sounds like voice. You can understand somebody on a terrible line in a windy lobby.

The door digit cannot take the same beating. The panel is not listening for meaning. It is measuring two exact frequencies, held for long enough to count. If either tone drifts, gets clipped short, or gets smeared by compression, the panel hears noise and does nothing at all.

Panel makers allow for this. A Mircom TX3 has a DTMF Sensitivity setting from 1 to 8, where 8 is the most sensitive and 5 is the default. It changes how readily the panel accepts a tone as a key press.

Anything sitting between your phone and the lobby that re-encodes the audio is a risk. Voice codecs are tuned to keep speech intelligible, not to preserve pure tones. That is why the same 9 that works on a direct mobile call can fail through a call-routing service.

What carries the door digit to your phone intact?

BuzzMeBro: the buzz arrives as an ordinary phone call, so the tone travels the normal path

BuzzMeBro gives your apartment its own local phone number. You hand that number to your building once, and they store it in the lobby entry panel like any other number. When a visitor buzzes your unit, every phone registered to that apartment that is set to ring is called at the same time, up to 4. Whoever answers first gets the call and can press the digit to open the door.

A buzz reaches you as an ordinary inbound phone call. There is nothing to install to receive one. Setup happens once on the website. The price is $9.99 per month or $95.00 per year, billed per apartment rather than per person.

Two limits worth saying out loud. A local number is checked at signup, and a building with no number available goes on a waitlist. And the door digit stays whatever your building programmed, because that setting lives in the lobby panel and not in your phone number.

One person, one working cell number: change nothing

If you live alone, the panel already calls your mobile, and 9 opens the door, leave it alone. Nothing here improves on that. A direct mobile call is about the cleanest path the tone can take. Read how apartment buzzers work if you only wanted to understand the plumbing.

Google Voice: free, but test the door digit before you rely on it

A Google Voice number can ring several phones, which is why people try it. The call takes an extra step through the service, and the two tones your keypad plays have to survive that step. If they arrive changed, you hear the driver, press 9, and the lock does not move. Test it once from the lobby before you rely on it. Google Voice and apartment buzzers goes through it.

A free forwarding trick also leaves the other half untouched. The panel holds one phone number per apartment, so your roommate still cannot let anybody in. A number that rings everyone in the apartment at once fixes that part.

What is worth checking on your own buzzer?

1. Find out which digit your building actually uses. Ask the property manager or check your move-in package. Do not assume 9, and do not assume the side door matches the front door.
2. Find out which number the panel calls for your unit. Buzz your own apartment from the lobby and see which phone rings, or work through find out what number your buzzer calls.
3. Check that the buzz reaches you as a plain phone call. Anything re-routing the audio in between is a step the tone has to survive.
4. Decide who in the apartment needs to be able to open the door. One number on the panel can ring more than one phone, and that is what BuzzMeBro does.