fax won't send from my tr4720, it just beeps and says communication error.

Diagnosis

The "Communication Error" accompanied by a beep on the Canon PIXMA TR4720 indicates a failure during the "handshake" process. This occurs when the printer attempts to establish a physical connection with the receiving fax machine but fails to receive a valid fax-ready tone. Essentially, the printer is dialing the number, but it is either not reaching the destination or the signal is being interrupted by digital noise. This is most commonly caused by the use of VoIP (Voice over IP) services, such as Xfinity Voice, Spectrum, or Vonage. Digital lines compress audio data, which often strips away the precise frequencies required for legacy fax protocols. Additionally, plugging the phone cord into the "EXT" port instead of the "LINE" port prevents the printer from accessing the external network, as the EXT port is designed solely for attaching a telephone to the printer, not for incoming service.

How to Fix It

To resolve this communication failure, follow these precise steps to isolate the hardware and configure the transmission speed:

1. Verify Physical Connections: Locate the two RJ-11 ports on the back of the TR4720. Ensure your phone cable is plugged firmly into the port labeled LINE. If the cable is in the EXT port, the fax will never send. Use a standard 2-wire phone cable; do not use a coaxial or ethernet cable.
2. Test for Dial Tone: Unplug the phone cord from the LINE port and plug a standard analog telephone directly into that same wall jack. Listen for a clear, steady dial tone. If you hear static or a digital hum, the line is too noisy for faxing.
3. Adjust Transmission Mode: On the printer's home screen, navigate to Setup > Device Settings > Fax Setup > Transmission. Change the setting from "Auto" to "T.38" if you are using a VoIP line. If that fails, switch it to "Standard."
4. Lower the Baud Rate: In the same Fax Setup menu, locate Fax Speed. Change this from "High Speed" to "Half Speed" (9600 bps). High-speed faxing often fails on modern digital lines; slowing the speed increases the likelihood of a successful handshake.
5. Check Dialing Prefixes: If you are in an office environment, ensure you have entered the required outside line prefix (e.g., dialing '9' or '0' before the number). Navigate to Fax Setup > Dialing Settings to verify the prefix is correctly applied.

If That Didn't Work

• Disable Call Waiting: If your phone line has call waiting, the "intrusion" tone from a second incoming call can disrupt a fax handshake. Disable call waiting through your service provider or use a dedicated fax-only line.
• Bypass Power Filters: If your phone line is plugged into a surge protector or a DSL filter, remove it. Plug the line directly from the wall jack into the TR4720 LINE port to eliminate signal interference.
• Test with a Known-Good Number: Send a test fax to a verified service like an online fax testing number. This determines if the issue is with your printer's hardware or the specific recipient's machine.
• Perform a Hard Reset: Power off the printer, unplug the power cable for 60 seconds, and remove the phone line. Plug everything back in and restart. This clears the internal fax buffer and resets the modem handshake protocol.

Heads Up

• VOIP Warning: Many modern "digital" phone lines are fundamentally incompatible with traditional faxing. If you use a cable-provider phone, you may need to request a "True Analog" line.
• Ink Levels: The TR4720 will often refuse to initiate a fax transmission if the ink cartridges are completely empty or missing, as it cannot print the confirmation page.
• Cable Quality: Replace old, frayed, or extremely long phone cords. Any cable longer than 15 feet can cause signal attenuation, leading to the "Communication Error" beep.
• Paper Jam Check: Ensure the Auto Document Feeder (ADF) is clear. A microscopic piece of torn paper in the feeder can trigger a sensor error that prevents the fax from sending.