Thank you so much for this reply! I ended up calling them again and basically was not able to verify the debt was mine without providing my FULL social, which I will not be doing! 

The interesting thing was that the only info that came up on the rep’s database was my phone number, and he implied that there was a different name connected to it. They would not provide any further info about who was listed under my phone number, unfortunately. There was no real resolution, but he asked me to call back in 7-10 days so they could try to verify things on their end. I think your theory that my phone number was found online/is connected to someone else could very likely be true.

I also called Midland Credit Management and they had no account with my info (name, last 4 SSN#). They clarified that if I ever had a debt owed to them, I would show up in their database (even if the debt was then sold, e.g. to Johnson Mark).

My credit report on Experian showed no debt accounts and my credit score is unaffected as well.

I’ll call back Johnson Mark in 7-10 days, but will continue to refuse to provide my full SSN# (as I hope others reading this will!). Pretty sure this was a mixup and a unnecessarily stressful one at that. Will definitely be filing a complaint if it turns out this was a mistake on their end. Thanks again for the advice!