Send a Debt Validation letter to Midland, make them prove you owe the debt and that they are legally able to collect upon this. There is a chance, if it is old, that southwestbell and ATT no longer own it, which is why you never heard anything from ATT. Either way, make Midland prove you owe it and that the amount is correct.
If they can prove it, then offer them a pay for delete which is you pay, they remove it from the credit reports. Make sure any deals you get in writing. Always send everything certified mail return receipt.