It's not and well within your right to demand Debt Validation.I am definitely taking it seriously because I don't want it to have a negative impact on my future credit, but I'm not just going to give up $3,000 to some guy over the phone if he won't tell me why. He already had my information. He knew my name. He knew my phone number. He knew where I worked. I would have been willing to talk to him and figure it out, but I literally had to leave for work. He was making me run late. Even if this debt is legitimate, I refuse to do anything about until I get something in writing proving that it's real. I don't think that is unreasonable.
If you're not planning on opening any line of credit soon, I'd pay the $7 and put a security freeze on your credit report with credit reporting agencies.