I do hope you did NOT give him your son's credit card number - per the Federal Debt Collection Practices Act (FDCPA), the debt collector must MAIL an initial dunning letter to you within five days of their first call.  Refusal to mail any proof of debt means that you should not pay!  Anyone can call and threaten someone about a debt - that does not mean the caller actually has the right to collect said debt (if it even exists).   You may wish to read more at the following link about what debt collectors can and cannot do:

https://www.consumer.ftc.gov/blog/facing-debt-collection-know-your-rights

Dealing with financial trouble is stressful enough. But when a debt collector is hounding you using unscrupulous methods, it can make matters even worse.

The fact is, you have some rights when it comes to dealing with debt collection agencies. They’re spelled out in the Fair Debt Collection Practices Act (FDCPA), which  protects consumers from abusive, harassing or unfair debt collection practices, and is enforced by the FTC. Here’s the abbreviated version of the key provisions:

Under the FDCPA, debt collectors are not permitted to:

call you before 8 a.m. or after 9 p.m.
contact you at work if you’ve told them verbally or in writing that your employer doesn’t allow you to get such calls in the workplace
contact a third party about you for any reason other than getting your contact information; simply put, they may not tell anyone that you owe money
harass or abuse you or anyone else they contact about you
lie about the amount you owe
use deceptive methods to collect a debt from you. For example, they may not:
falsely claim to be law enforcement officers
claim that you’ll be arrested if you don’t pay your debt
threaten to seize, garnish, attach, or sell your property or your wages — unless they are permitted by law to do it and intend to do so
give false credit information about you to anyone, including a credit reporting company
use a fake company name

You may also wish to contact your state's Attorney General for help.

There is no reason to explain anything to these crooks.  Make them follow the law.  They won't.  I can't find this so called debt collector listed anywhere.  What you need to do is to get an address for them if possible.  Post it here for us to research.  They are now required by law to do this:

Federal law (FDCPA) requires them to send you a letter (US MAIL ONLY) postmarked within 5 days of their first contact that contains their name, physical address, the creditor’s name, and the amount of the alleged debt.  It also must contains “mini-Miranda” telling you that it is an attempt to collect a debt and that all information will be used for those purposes.  The one other important thing that this letter must also have in it is that you have a right to dispute the debt within 30 days of receipt of the letter and if you do so, all collection activity must be stopped until the debt is verified.

If you can get an address you can follow up:

You need to send them a certified, return receipt debt validation, cease and desist calling, or "not me" letter (think of it as the best $6.47 you ever invested).  You also need to report them to the proper agencies to which I have provided links to.  Then you need to go out and get a consumer lawyer to sue them on a contingency basis (no money out of your pocket) and let them pay you for the harassment.  If everyone did these three things, these crooks would be bankrupt and we would all be the richer for it.  Use these web sites to find an attorney in your state:  http://www.consumeradvocates.org/
http://www.consumeradvocates.org/find-an-atto ... tates_value=All

Read up on your rights here, get template letters to send and also make a complaint at this government site: http://www.consumerfinance.gov/

Also file a complaint with your State Attorney General's office.
List of State AG’s offices:  http://consumerfraudreporting.org/stateattorneygenerallist.php

I got a call as well from Norm Keiffer, however I didn't return the call. I have an ambulance bill from 2010 which I refuse to pay because it was work related,  but that's the ONLY item I have in collections. From what I'm reading this company sounds very unprofessional and I'm guessing they're fraudulent. I googled AR online and found this link http://accountsreceivable.com which looks legit. I think Norm is probably pretending to work for this company to access credit card or bank info.
If I were I'd contact the company in which your son owes and disregard this company.