The message "clear toner counter" will display even after replacing the toner cartridge if you did not complete steps 9 and 10 of the procedure "Installing the Print Cartridges" detailed in the fax machine's manual. A link to the manual:
http://www.sharpusa.com/files/fax_man_5700.pdf
Step 9
Reset the toner counter by pressing
the LIFE key (flip up the Rapid Key
overlay), 2, and the START key.
Step 10
Reset the drum counter by pressing
the LIFE key (flip up the Rapid Key
overlay), 3, and the START key.
I do not know if clearing the message will enable you to send a fax, but following the instructions from the manual will clear the message you are getting.
Dustin Ludwig, April 2007