If you receive an “authentication failed” message when you try to connect to the Internet using a dial-up account to your ISP, your user name or password may be entered incorrectly in Network preferences.
Choose Apple menu > System Preferences, then click Network.
Select your modem in the list at the left.
If you don’t see your modem in the list, make sure your modem is connected to your computer (internally or externally), click the Add button at the bottom of the list, click the Interface pop-up menu, then choose Modem. Give the service a name, then click Create. If there are no settings, enter the telephone number, account name, and password for the service.
Make sure your account name and password match those given to you by your ISP.
If your current password is not shown, carefully retype it to make sure it is correct. Don’t interpret the number of dots in the field as the number of characters in the current password; the numbers may not match.