If I were in a business situation like that over a substantial amount of money, and really felt I owed nothing to the purported creditor and they had no leg to stand on, I'd call my business attorney.
But if I didn't have the money for that, I would write a letter demanding evidence that I owed the money and the collection agency "owned" the debt and was authorized to collect it, and that I would consider a failure to respond within ten days a prima facie admission the debt was not valid. Then I'd send it certified mail, return receipt, and keep a copy and staple the certified mail receipt to it, and add the return receipt when it came back. That would give me a paper trail in case things went any further, and the return receipt puts the collection agency on notice that doing exactly that.
My wife and I have both gotten bogus collection letters I've dealt with that way - hospital visits we never made, etc. In each case, the collection agency has dropped the attempt immediately. A very nice lady at one of them admitted that sometimes, when somebody just skips out on a bill, the agency finds other people in the area with the same name and hopes somebody will be gullible enough to pay it.
If they threaten a suit, and you're really confident there's no credible reason you'd owe the money, you could mention malicious prosecution. I found out about that from a guy who was sued by a collection agency over a business deal, and the person who referred it to the agency didn't give them all the facts. The guy actually wasn't on the hook for the debt and gave credible proof to the agency, but they proceeded to trial anyway. It was an open-and-shut case decided in his favor, and his lawyer told him he could countersue to recover malicious prosecution damages as well as legal fees, but he decided to just let it go.