Brother DCP-L2520DW Basic User's Manual page 68

Difficulties
Dialing does not work.
(No dial tone)
Poor sending quality.
Vertical black lines
when sending.
Suggestions
Check all line cord connections.
Try changing your resolution to Fine or S.Fine.
Make a copy to check your machine's scanner operation. If the copy
quality is not good, clean the scanner.
For more information, see the Online User's Guide: Telephone and Fax
Problems
Black vertical lines on faxes you send are typically caused by dirt or
correction fluid on the glass strip.
For more information, see the Online User's Guide: Clean the Scanner
53