Why is AT&T not letting me log in to my bank account? ?r=1247158615?

Are you using AT&T security suite?
This may be a cookie issue...
Try this:
Allowing site specific cookies
Depending on what browser you use;
Firefox> Tools> Options> Content: here, check 'block pop ups', then tick 'exceptions'> (enter the URL needed for permission) > 'Allow' > OK out.

IE: Go to the site first> Tools> Internet options> Security> tick the 'Trusted' icon> Sites> Add> ok> out
Note this setting may permit undesirable 'Active scripting' or other server based action: use with caution, especially if you have 'Active content' allowed.

Supplemental blockers, in browsers mainly, may have to be muted, or 'site allowed'.
"NoScript' users will have to "temporarily allow....." from the blue 'S' tray icon.

No need to relax any other cookie settings (which help deter junk ads, pop-ups', malware etc.):
FIREFOX: Tools> Options> Privacy: Check 'Accept Cookies from sites'
& **Un-Check 'Accept 3rd Party cookies'**
INTERNET EXPLORER: Tools> Internet Options> Privacy> Advanced:
here check 'Override automatic....'; 'Allow session cookies'; 'Allow 1st party cookies' & **'Block 3rd Party Cookies'**.

Or possibly the site will have to be added to your 'trusted' sites, perhaps in the Security section of AT&T settings.
Hope this helps.