It's extremely unlikely that you owe Paypal any money unless you've been hiding for 10 years - the Statute of limitations would extinguish any debt by now.
However, find out who made the call - there are some rather unscrupulous credit agencies that buy up old, written-off debts and then hound people for the money, hoping to get lucky.
If you actually DID owe money, it's possible that this debt was legitimate at one time. It's also possible that someone with your name has incurred the debt and the collection agency is pursuing you in error for it.
If they call again, you have a right to insist on proof of the debt. Do so. Also tell them that they may not telephone you again about it and any discussion must be in writing.
If they place a note on your credit file, there are things that you can do, but you'll have to research them for your specific jurisdiction. Use Google.
