This is the same question I answered a little while back - but now at least you have provided more helpful information.... If what you say is true, that you have everything connected properly, and have established a dial-up connection - and AT& T says it looks good from their side, then you may have a virus on your computer that has re-directed all of your internet traffic to an invalid proxy that is preventing you from connecting. First, establish that you in fact can connect by opening a command window (click start and type cmd into the box and press enter) then ping google e.g. ping www.google.com - to see if your dns is working. You should get results like this...
Pinging www.l.google.com [220.127.116.11] with 32 bytes of data:
Reply from 18.104.22.168: bytes=32 time=1266ms TTL=43
Reply from 22.214.171.124: bytes=32 time=1794ms TTL=43
Reply from 126.96.36.199: bytes=32 time=1819ms TTL=43
Reply from 188.8.131.52: bytes=32 time=1722ms TTL=43
Ping statistics for 184.108.40.206:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 1266ms, Maximum = 1819ms, Average = 1650ms
If this works, then open a browser and type in the ip address only 220.127.116.11 and google should come up.
Check your internet settings to make sure no malware has set a spoof proxy address for your connections. Also, in your connection window (Tools -> internet options -> connections) make sure you have the correct options (e.g. dial-up settings) check your lan settings to make sure autodetect settings is set. If the ping command worked (above) then you are truly connected and the issue is in your browser settings - if not, then it's a connection issue - check the errors in the event viewer under control panel - administrative tools - event viewer - that may give you some insight into what is going on. Your modem could be bad - and remember, you need to connect to the internet first - then try to open a web page. Good luck.