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Yes it sounds like an old debt to me, because of the 50%. You can ignore them if you wish or send a cease and desist letter.
You can use the search option, to find cease and desist letters
Once they made any intiail communication with you, that triggered their requirement to send you a written collection (dunning) notice within 5-days, which, among other things, must identify the original creditor. Have they done this?
If they call again, just tell them to direct all further communications to them in writing. It is your right to know for whom they are collecting before you have ANY discussion with them.
If you, by chance, have their correspondence address, you might send them a DV letter.