Have you attempted to contact AT&T to see if there is something they can do to unblock this? There are a few other options you can choose from to resolve this issue. Basically you just need to change the ip address it is connecting through and that would resolve the issue as they are more than likely blocking the ip address you are on. You can do this with business hosting, a dedicated ip address, or even a cdn/firewall. Anyone of these options would change the ip address so that it would no longer be blocked.