Midland Credit Management

I recently received many settlement offer from midland credit management, Inc. It has MCI listed at original creditor. It is also listed on my credit report under Midland Credit Management. I recently found out that MCI put a balance in collection in 2002 without me being aware until last summer. I had been MCI customer for 5 months during 1999 and then I paid in full the balance indicated in the final bill. An year later a moved out that apt and I knew nothing about MCI still claiming a balance on my name. I called the previous collectors to know what the story was and it was like waking-up a sleeping dog, indeed Midland Credit Management started harrassing me with phone calls every day, even 2-3 calls per day. I had to change my number. I did not respond to Midland first letter in which they asked me if I want to dispute the debit. Now they keep sending settlement offers. What should I do? Can I still dispute the debt? what is gonna happen if I keep not paying? Thanks for your attention