Long time DNS administrator here...
It is theoretically possible that the ATT DNS server still has the old forum IP address cached.
I don't know specifics about how they transitioned the forums, but there are a few things you can do that will leave the old IP addresses in cache across the internet. If that happens, you sort of just have to wait until the cache values time out. No DNS server admin is likely to flush caches on their servers for sites that change IPs. (If you did, you'd be doing nothing all day long but this.)
With a command line, these sorts of things are easy to check... but not entirely sure what tools are available on your phone. If you know the ATT DNS server IP(s), I can attempt to poke at them but:
1) I may not be able to hit them off network
2) it's more than likely that they're using load balancers or anycast IP addresses such that even if I knew the IP... I'd be hitting a different server than you
---geeky details you probably don't care about-- --You can stop here if you want.--
Every DNS "question" gets an answer with a TTL (time to live) value in seconds. You can specify it as almost nothing (1 second) or specify it as up to a week long. When a DNS server (like ATT's DNS server) queries the DNS server for mmm.com... it will get the answer and cache it for as long as the TTL says to cache it.
When you transition a site... it's possible the DNS is set high. Before you transition, you need to set the TTL to something low (like an hour)... and then wait. In other words, if your TTL is a week, you need to set it to an hour a week in advance... so the cached values won't be sitting there.
Now: I'm not saying that's what happened. I'm just saying... this is something that happens commonly. When it does... you pretty much just have to wait.