Some AT&T U-verse customers are still without TV, Internet, or phone service after a days-long outage that knocked out access for hundreds of users throughout several Southern states.

"The issue impacting some U-verse subscribers has been tracked back to a software upgrade," the company wrote on its Facebook page earlier this afternoon. "We continue to work to determine when service will be completely restored. Our continued apologies for the inconvenience."

The outage began on Monday and was confirmed by AT&T on Tuesday. Hundreds of users have complained of no service on the company's online forum under the thread "U-verse outage -- what's going on?"

"I have been without TV, Phone, Internet for almost 5 hours now," one person wrote on Monday shortly after the outage began. "I'm using my cell phone to post this."

Another user wrote earlier today, "Owasso OK (near Tulsa) back on with everything as of 16:00 today. Total down time 40 hours. I don't recall ever having a longer outage on any cable, Sat or internet provider before. This was (still is?) a serious problem and I for one would like to know more about what happened for my own piece of mind. AT&T is too big and important to our national infrastructure."

Although it's still not totally clear what happened, AT&T has said that the downtime was due to a faulty software upgrade. Initially, the company thought it had to do with issues with its servers but then realized it was because of the upgrade.

AT&T said on its Facebook page that the issue affected less than 1 percent of its 7.4 million U-verse subscribers. Despite the issue being resolved for many people, some users are still posting on the AT&T forum that their service is yet to be restored.

Updated January 24 at 6:00 p.m. PT with comment from AT&T spokesman Mark Siegel:

"U-verse service has been restored for all customers affected by the outage. The software problem causing the issues was resolved by AT&T engineers early this morning. We are not pleased it took so long to fix the issue. AT&T will provide a credit to customers who were affected. We know our customers count on their U-verse service and we apologize for the inconvenience."