Ive used ATT DSL for several years. 99% of the time it is good.
The 1% is what we are talking about and Ive been thru your exact situation.
Resolution took ~45 minutes. Call ATT tech support and make sure you are up and running online. That usually requires a direct to your PC connection.. ie no router. Once you have confirmed you are online time for the next call.
Linksys's tech support I have found to be very good and fast. They will walk you thru the entire procedure and get you running. They are patient and knowledgeable.
If you want to increase your speed and lower your cost ... here is a good website to read. I have used their advice 3x and and saved a bunch. Last time I used it .. it was for 2 phone lines and the total saved is about $540 a yr.
http://slickdeals.net/forums/showthread.php?t=1146593&page=115