Unless you are well spoken, and very confident in your knowledge of the collection process I would avoid speaking to them.
Traditionally, NCO has had suit filed for abusive practices and intimidating people into providing personal identifiable information they are not entitiled to making their personal situation worse.
As far as the phone calls, I would write to their compliance manager Lisa Signore. Her direct email can be found on the Better Business Bureau website. Inform them you want all telephone calls to be discontinued immediately and demand they communicate with you in writing as to the purpose of their harassing phone calls.
It is illegal for them to phish for personal information such as your DOB and SSN. They are very easily able to bring up the account information by the telephone number they are calling.
Pull your credit reports. This may give you a clue. NCO is notorious for putting information on the credit report. By law, if NCO reports a collection they have to state who the original creditor is and the dollar amount.
If the trade line is not reported, escalate your dispute to the BBB and the PA Attorney General solely on harassing phone calls and their illegal phishing for personal information.
If the trade line is reported, contact the original creditor to see if the account is yours, if NCO is legally contracted to collect. If you do owe the bill - PAY THE ORIGINAL CREDITOR and not NCO. NCO has been sued for illegally pulling funds from consumer acocunts without their persmission. You don't want NCO getting your personal information. If you must pay NCO direct and there is no way around it, use a money order to protect your personal information.
It's a process to sort this out, but your first step is getting the details.