IQ Data International review: showing no proof of debt 1

I had cosigned for a friend of mine to get an apartment in 2009. He left in 2010, then i was hit with a collections debt of 2400 of "damages" to the apartment. IQ data international are the ones handling this matter.. 8 months ago i disputed this with experian and i won. This account was removed because they failed to show me proof of receipts and pictures of the "damages"... Now 2 days ago a fellow name Doug Jones started calling me harassing me telling me i have to pay or they are going to sue me. I said i already disputed this and he said they are going to put it back on my credit. That i am going to get sued... I asked him to mail me the proof of the debt and he responded "we don't have to", "pay your bill!" i felt disrespected and belittled.. i simply hung up! This company is known to not want to show proof but demand us to pay! makes no sense. I am a citizen of the United States and no citizen should get treated the way they treated me. Please Help...