Can not send faxes, but can receive them

A voice over Internet protocol device, magicJack allows you to make and receive telephone calls from your computer using a USB device. While magicJack does not officially support sending and receiving faxes, you can connect a regular fax machine to the magicJack device to send and receive faxes.
Attach the magicJack device to a USB port on your computer.
Connect the phone wire to the port at the end of the magicJack device.
Connect the other end of the phone wire to the "Input" jack on your fax machine.
Turn off error correction on your fax machine, and set your fax machine to the lowest possible transmission speed. Consult your fax machine's user guide if you do not know how to turn off the error correction or lower the transmission speed on your fax machine.
Send a fax to your magicJack phone number to receive a fax. You can find your magicJack phone number at the top of the magicJack splash screen that will display on your computer after inserting the magicJack device.
Send a fax from your magicJack by loading the fax you want to send into your fax machine and pressing the "Send" button.

Error 344 means :-

The remote machine has failed to respond to a fax command from the product because the connection is interrupted.
The remote user has pressed the Stop button while the remote machine was receiving.
Under rare conditions, incompatibility between the two machines can cause the remote machine to simply abandon the call.

Solutions :-

1. Attempt to send the fax again when telephone-line conditions have improved.
2. If the fax session was using error correction, disable ECM.

To disable ECM Press :-

1. Press Setup .
2. Press the Down arrow ( ) repeatedly until Advanced Fax Setup displays.
3. Press OK .
4. Press the Down arrow ( ) until Error Correction Mode displays.
5. Press OK .
6. Press the Right arrow (> ) until either OFF or ON displays.
7. Press OK .
8. Send another fax to verify that the ECM is enabled or disabled.

Check the Phone Cable connected with the printer.
Make sure the Cable should be connected to line 1 port at the back of the printer and it should be going directly from the printer to the wall jack and not to the Serj protector.

Try to print a fax test page.

1. On the control panel of the product, press the Setup button ( ).
2. Press the Right Arrow button ( ) to select Tools , and then press OK .
3. Press the Right Arrow button ( ) to select Run Fax Test , and then press OK . The product displays the test results on the control panel display and prints the Fax Test Report.

Now Do this :-

1. Send a one page black and white text fax to 1-888-hpfaxme (1-888-473-2963).
2. When your fax is received by the HP Test Fax Service, we will generate a return fax to you within five minutes confirming that we received your fax. This verifies that you can both send and receive a fax with your fax machine.

If you do not receive a return fax within 5-7 minutes, there may be a problem with the setup of your fax machine. Consult the user manual that came with the product for proper setup and configuration. Try the above steps and let me know if it works for you.

This is because some faxes don't have Error Correction Mode capabilities.

If there's a slight distortion on the line, the receiving machine (without ECM) receives a fax with some minor errors. And even though for a human eye the resulting fax page is perfectly readable, for the machine, that thinks only zeros and ones, there was an error detected in the data, and hence the trasmission failed.
Faxes with ECM when they detect an error, ask the sending fax to re-send the last portion of data, and this way can bring the transmission to a 100% successful end.

So the only thing you can do is ask your recipients to upgrade their fax machines to an ECM-capable model :)

The receiving fax machine is turned off or not functioning. Try calling your cell phone.

Do you have a DSL phone line?

Make sure you are plugged into the LINE port of the fax machine.

First, check your fax's phone line. Put a regular phone on it and check for dial tone and line noise. If the line is dead or noisy, this will cause a problem. 
You can also try dialing your fax number from another phone and see if the fax unit picks up the line and starts initiating communication. (sounds like modem noises)

error 634 is an indication that the line was either busy, or there was no answer on the receiving end.

You can not send or receive faxes with any consistency on Broadband lines. You need an analog telephone line for these to work. Fax machines work on analog technology.

When the last page of your document has been sent successfully, the machine beeps and returns to Standby mode. When something goes wrong while sending your fax, an error message appears on the display. If you receive an error message, press Stop/Clear to clear the message and try to send the document again. You can set your machine to print a confirmation report automatically each time sending a fax is completed. If you get the error message write it down it should always supply an error message if the fax did not go through.

Try this website: http://www.instruccionesweb.com/u2/pdf/fax/Canon-L120-en.pdf