By all means you should sue them in District Court. In District Court you can be awarded up to $1000 in statutory damages for their violating the Fair Debt Collections Practices Act. You can also be awarded reasonable attorney fees. What's even MORE important is that you can sue for ACTUAL DAMAGES. Since you are in the process of financing a mortgage, ACTUAL DAMAGES becomes readily provable, thus increasing the award you're likely to win.
Further, you can also sue them for violating the Fair Credit Reporting Act.
What to do: 1. Contact an attorney in your area that knows consumer credit laws-many law firms are now SUING these bad collection agencies. File formal complaints with your State Attorney General and THEIR State Attorney General. Also file complaints with the FTC. Also, go to the FTC website, www.ftc.gov and download the FDCPA and the FCRA. If you can get ATT to verify in writing that there was the 3 way call and that you do not owe this debt, all the better for you when you SUE THEM.
2. Send them a certified, return receipt requested letter that explains this is not your debt and that you expect them to contact the three credit reporting agencies to have this false debt line deleted immediately.
Good luck. Don't waste any time. The statute of limitations for you to sue them for FDCPA violations is 12 MONTHS from the time that the violation was committed.