I would assume they have tested and have a good idea of where the issue is, which is why they reported it to the Telco side. The issue the Telco side have could be anywhere in their system, the C.O., the lines between, repeaters etc, not necessarily in your building. I would talk to the PBX side of AT&T and ask them for updates and tell them this is service affecting and you want updates every hour until its fixed.