It's actually an automated call center. If you don't answer the call, an automated system leaves you a message. You may get calls from 505-797-6322, 505-797-6140, etc... basically any 505-797-6###. I thought it might have been a scam, so I tried resolving it directly through a number I found on Citi's website, but even after speaking with a supervisor (multiple times), it didn't get resolved. Finally, one of the supervisors asked if it was a 505-797-#### number. Once I told her it was, she then gave me the number that I should call 505-797-6321). Once you go through the initial language prompt, I believe I selected option 2 to speak to someone. I then explained that I did not have any Citi accounts and I wanted the number removed. You DO NOT need to provide any SSN, just your name and phone number. The calls then stopped a couple of days later.
Here's how I was told they got my number. Citi gave an initial name to call for collecting the debt. The collection company then started calling every phone number that matched that person's name in an attempt to get to the correct person. My cell number must have still had some old information on it from the previous person. Very annoying, but the calls stopped a couple of days later after I made the call to 505-797-6321.