Open up Audacity by double-clicking on the program icon. If you don't have it, download it (see Resources).
Open the phone recording you'd like to clean up by clicking on 'File' and then 'Open' and selecting your audio file from the directory list. Audacity can open sound files such as WAV, AIFF, MP3, AU and OGG. If your sound file has a different file extension than those listed, you must convert the file to one of those by using conversion software before using Audacity.
Move your cursor to a section of the audio that does not contain any voice recording. You can tell what area this is because when you look at the audio wave, you'll see only a single line (with no peaks or valleys). Click and hold the left mouse button and highlight a small area of your "silence."
Click on "Effects" and select "Noise Removal." When the Noise Removal window opens, click on the button in the top half of the window labeled "Get Noise Print." The program will automatically generate a reading of the noise contained in the area of your sound file that should be silent (where you set the cursor). This noise is what is present throughout the entire file. Close the Noise Removal window.
Highlight the entire sound file by clicking your left mouse button in the window (at the beginning of the file) and holding the button down while you drag your cursor all the way to the end of the file. Release the mouse button and the entire sound file will be highlighted.
Click on "Effects" and select "Noise Removal." When the Noise Removal window opens click on the button in the bottom portion of the window that is labeled "Remove Noise."
Listen to your file. If you still hear line noise, go back and repeat this process. It will sometimes take two or three attempts before the line noise is completely erased.