This is definitely not the right forum for your topic.
Within 30 days of first contact, you must mail a debt verification request letter to dispute it. Send it certified mail with return receipt requested. At that point they cannot continue to try to collect until they provide proof.
It has to be done in writing and there is no denial option for them. They must legally comply with this process. I mean if you are located in the US, and you might want to check your state laws as well, for statute of limitations on debts if nothing else.
Even if you dispute a debt, that does not stop them from pursing it after they send proof, nor selling it to another collection agency but then this new agency must still handle your dispute letter by supplying proof. I mean you must mail the letter to this new collection agency.
There are different ways to respond appropriately to debt collectors. Depending on your situation, we have sample letters you can use if you're experiencing common problems.
www.consumerfinance.gov
Keep in mind that
I am not a lawyer, but many lawyers will give you an initial consultation for free, and look on the bright side... the more times this debt gets moved to a different collection agency, the more likely they will lose vital info needed to pursue the collection, or win in court, but if they do sue, be sure to go to court to deny the debt (I don't mean lie, just state your case) or else you'll get a default judgement against you, even if they claim wrong dates and amount of debt.
It may be legal for the collection agency to add on interest and fees to your original debt, if the total is higher than you think it should be. Again, seek a free consultation with a lawyer.