I’ve had collectors come after me a few times. I instruct them to contact me in writing (as required by my state law) and follow up with a registered letter, and I log every time they call me or leave a message. The legitimate ones do this, and get paid; the scammers don’t.
Eventually it comes down to a phone call where they threaten to garnish my wages (for a debt they can’t establish that I owe, and won’t contact me in writing about) and I threaten to bring the copy of the certified letter and a history of the phone conversations to the attention of my state’s attorney general. That usually ends the matter.
There was a scam that they tried on me a few years ago – they call claiming you may owe their (unnamed) client money, and the agent on the phone is willing to offer you a deal right then and there. The debt is always about $1000, and they offer to settle for $200—$300, which seems reasonable; but they don’t allow you time to actually check that you owe the debt. I can only imagine that there are some people so terrified of debt collectors that they just hand away a couple hundred dollars, but I’m not one of them.
These people make their money off of intimidation and fear. Don’t give in.