Here are a troubleshooting guide Flash movie from AT&T and Yahoo.
It is a good habit to review the whole troubleshooting movie. When you meet the similar problems, it may guide you to fix your connection.
This is based on the AT&T Yahoo! Help file.
To create a new DSL connection profile for Windows XP:
The Windows XP operating system includes its own PPPoE software. You should use it instead of Enternet 300.
1. From the Start menu, select Control Panel.
2. Make sure you are viewing the Classic View. If instead you are in Category View, select Switch to Classic View from the upper-left of the window.
Devices such as fax machines, caller ID boxes, or telephones that connect to a DSL phone line require a line filter, which prevents noise from disrupting the DSL signal on the phone line. Line filters must be added to every device that connects to a DSL phone line. Note: The line connected to the DSL modem can connect to a dual stand-alone filter. Otherwise, do not use a filter on the DSL modem. There are two basic line filter configurations available. First determine if you have dual stand-alone line filters (figure A), or single stand-alone line filters with a “Y-adapter” (figure B). Then follow the appropriate set of instructions below.
Setup instructions for dual stand-alone line filters (figure A)