ATT Wired Phone Line for Business Security System - Cannot access account

I have an ATT wired phone line for our business security system and I have been having some reoccurring issues with the line going down and affecting our security system & it not being able to arm because the line is down. When I call ATT I seem to keep hitting dead ends because I don't have our account number OR any invoices. We haven't received anything from ATT. I checked with our security system company and they do not handle the phone lines, it is a line we have to provide. I have been trying to track down the information for our account but all I have been getting is a run around. I don't know how to fix an issue when I can't even get support for said issue because no one will help me track down the account and set up billing to our physical address so we can receive invoices. I keep being told that they cannot acces anything without an account number and when I let them know I do not have an account number, I then am told to give them the exact dollar amount of our last invoice. Can't do that either as we haven't received any invoices. It's getting to the point where we are considering looking elsewhere for services as this has been happening extremely often and we've had four different instances in the last three months with the line being down and not being able to do a thing to even access our account. At this point I am so frustrated I don't know what else I can do.