It IS bull. Totally. I doubt this "Sgt." is really a sergeant of anything and next time they call you should ask them how would law enforcement like the idea of someone impersonating an officer. Also, you CAN'T go to jail and it was completely illegal of them to even say such a thing.
Do NOT pay them one dang penny! Tell them to send you a letter, through regular mail, that they are in fact required by law to send this letter within 5 days of first contact and that you will be requiring them to validate the debt.
If they try to say you already admitted to the debt you tell them you were confused and scared because of the illegal jail threat and felt intimidated to say what they wanted to hear.
Please do not pay a voice on the phone. By law they have to prove the debt.