Why is "Append to Log" Disabled?

If you are trying to perform the Windows Modem Log Test, and your "Append to Log" option is disabled, then you are running without sufficient permissions to turn on logging.  To correct this, make sure you are signed into Windows as a user with Admin permissions.  Use Windows Explorer to browse to "C:\Windows\System32" and look for a file "telephon.cpl".  Right mouse click on telephon.cpl, and choose "Run as administrator".