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Call waiting interrupts faxes because the incoming-call beep breaks the fax tones your machine is sending, and the connection drops. The fix is to disable call waiting for that one call by dialing *70 (or 1170 on a rotary-style line) before the fax number. That turns call waiting off only for the call you are about to place.
A fax call is two machines holding a continuous audio conversation over the line. When a second call arrives, the phone company injects a short beep into your line to announce it. That beep collides with the fax tones, and both machines hear garbage for a moment. Most fax machines treat the corrupted moment as a line failure and hang up, so the fax fails partway through, sometimes after sending the first few pages.
This is easy to mistake for other problems, because the error message on the machine usually says something generic like "communication error" or "line failure" with no mention of call waiting. If your faxes fail at random points and always when the line is busy with incoming calls, call waiting is the first thing to suspect.
On most North American phone lines, dialing *70 before the fax number disables call waiting for that call only. On older rotary or pulse-style service, the equivalent is 1170. The full sequence looks like this:
Many fax machines and multifunction printers can do this automatically. Dig into the fax or phone settings on the machine for a "call waiting" or "disable call waiting" option; when it is enabled, the machine dials *70 in front of every fax number on its own. That saves you from remembering the code each time, and it covers speed-dial entries too.
Here is the catch the code cannot fix: you can only disable call waiting for calls you place. When someone faxes to you, the incoming call is under their control, and your line's call waiting still announces any second incoming call over it. The same beep collision happens, and the fax drops.
For inbound faxes on a shared voice line, the honest options are narrower:
| Option | What it does | Trade-off |
|---|---|---|
| Disable call waiting permanently | Call your carrier and ask them to remove call waiting from the line entirely | You lose call waiting on voice calls too |
| Dedicated fax line | Give the fax machine its own number with no call waiting attached | A second line to pay for |
| Ring pattern or distinctive ring | Lets one line carry a second number so you know which rings are faxes | Still does not stop the beep on the shared line |
| Online fax for sending | Send over the internet instead of the phone line | Does not help the machine receive inbound faxes |
One more detail worth knowing: on some phone systems the *70 code is dialed as part of the number string with a pause after it, like *70,5551234567, where the comma is a dialing pause. The pause matters because the machine needs to wait for the second dial tone before continuing. If *70 followed immediately by the number gives you a fast busy signal, add the pause; most machines insert it when you type a comma or hyphen in the stored number.
It depends on who controls the line. Traditional landlines and most cable-phone (VoIP) providers honor *70, though some call the feature a different name in their settings. Cell networks handle call waiting differently, and a fax machine hooked to a cellular terminal or an analog telephone adapter may not respond to *70 at all. If the code does nothing, check with your provider: some let you disable call waiting in an account portal instead of by dialing a code.
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