Make sure that the WAN IP on the Linksys router is not in the same network as the LAN network of the Linksys.
E.G.
WAN IP: 192.168.0.1
LAN IP Network (IP of Linksys to admin the Linksys): Default IP is 192.168.0.1
To check this, go to http://192.168.0.1 and log into the Linksys. Then check the Status of the Linksys and you will see the LAN and WAN IPs. If they are the same or even in the same network, you will have problems. If they are the same, go to the LAN settings and change the IP Range to something else like: 192.168.1.1.
Now the two ports are on seperate networks and everything should work just fine.
Also, make sure you have the link setup to use PPPOE connection. Then you will need to input your Username and Password for the DSL connection.
You may have to call SBC and have them walk you putting the modem in Passive mode. This will pass the public IP to the router. This will correct the above problem with the IPs. But I would guess that the DSL Modem is passing a DHCP IP to the router and they are in the same network.
If this doesn't help, post more information like IPs.