Fax machines have built-in phones because they share the same telephone line. In most offices and homes, running a separate phone line just for faxing was expensive and impractical, so the fax machine needed a way to handle both voice calls and fax transmissions on a single connection. The handset let you answer a ringing line, talk to the person on the other end, and then switch to fax mode when needed.
One Phone Line, Two Jobs
The core reason is economics. Traditional phone lines cost a monthly fee, and most small businesses and home offices didn’t want to pay for a dedicated fax line on top of their regular phone service. Plugging a fax machine into your existing line solved the cost problem but created a new one: how does the machine know whether an incoming call is a person wanting to talk or another fax machine trying to send a document?
That’s where the handset earns its place. When the phone rings, you can pick up the handset and listen. If you hear a person’s voice, you talk normally. If you hear a repeating high-pitched tone (a 1100 Hz beep that pulses on for half a second, then off for three seconds), that’s a fax machine announcing itself. You press the start or receive button, hang up the handset, and let the machines take over.
Some fax machines automated this process with a built-in fax/phone switch. The machine would answer, listen for that calling tone, and route the call accordingly: fax tones went to the printer, and silence or voice triggered the phone to ring so you could pick up. External switching devices like Viking Electronics’ line-sharing units could do the same thing, detecting fax tones, distinctive ring patterns, or touch-tone codes to route calls to the right device.
How Fax Machines Talk to Each Other
The international protocol that governs fax communication requires machines to announce themselves with specific audio tones before any document data is exchanged. A calling fax machine sends that 1100 Hz pulsing tone to signal it’s not a human. The receiving fax machine responds with a continuous 2100 Hz tone lasting roughly three seconds to confirm it’s ready. Only after this handshake do the two machines begin exchanging document data at 300 bits per second.
These tones exist partly so that a person who accidentally dials a fax number hears an obvious, sustained signal that tells them they’ve reached a machine, not a person. The protocol was specifically designed so “a normal telephone user who becomes inadvertently connected to one will receive tone signals for a period of sufficient duration” to realize the mistake. The handset on the fax machine gives the receiving end the same courtesy: you pick up, hear the tone, and know to let the fax take the call.
Manual Sending and Troubleshooting
The handset isn’t just for receiving. When sending a fax manually, you pick up the handset, dial the number, and listen. If a person answers, you can tell them you’re about to send a fax so they can press receive on their end. If you hear the answering fax tone, you press start and hang up. This person-to-person coordination was especially useful for sending to shared lines where someone needed to physically switch the machine to receive mode.
Troubleshooting is the other major use. Fax transmissions fail for all sorts of reasons: busy signals, line noise, incompatible machines. When a transmission report shows “No Response” or “Busy,” the standard advice from manufacturers like Brother is to call the recipient and verify their fax machine is working. Having a handset built right into the fax machine means you can make that call without walking to another phone, and you can test the line quality yourself by listening for static or crosstalk.
The Extension Port
Most fax machines have two phone jacks on the back: one labeled “LINE” for the wall connection and one labeled “EXT” for plugging in a regular telephone or answering machine. This daisy-chain setup let you keep a standard phone connected through the fax machine so you could use the line normally when no fax was coming in. Canon’s setup instructions, for example, walk users through connecting an extension telephone to the EXT port so they can answer calls that come in while the fax is alerting them.
This design reinforced the single-line reality. The fax machine sat in the middle of the phone circuit, monitoring all incoming calls. If it detected fax tones, it grabbed the call. If not, it passed the call through to your regular phone or answering machine. The built-in handset served as a backup for when the extension phone wasn’t nearby or wasn’t connected.
Why It Mattered More on Analog Lines
Fax technology was built for the old analog telephone network, where each call got a dedicated, unshared circuit from sender to receiver. This direct path meant fax tones arrived intact and on time. The handset worked reliably because it was using the exact same type of connection as a normal voice call.
Modern VoIP (internet-based phone service) changed this. VoIP compresses voice data to save bandwidth, typically squeezing calls down to 32 kilobits per second or less. Voice sounds fine at that compression level, but fax tones cannot be compressed. Fax data needs a full 64 kilobits per second, and even 1% packet loss or a delay of more than two seconds can kill the transmission entirely. Many VoIP providers still recommend keeping at least one traditional analog line for fax machines, postage meters, and alarm systems.
On a VoIP line, the handset on a fax machine still works for voice calls, but the fax function itself becomes unreliable. This incompatibility is one reason fax machines with handsets have largely been replaced by online fax services that skip the phone line entirely, converting documents to email attachments or cloud uploads instead of audio tones.