I wonder if debt contracts can ever be described as purely voluntary?
Corporations have limited liability - not covered in the Constitution. Debt
collection via foreclosure or (not so long ago) jail entails government.
Bankruptcy is allowed for some, but not others. Gambling debts may be
enforced by private thugs. Debt slavery was common through much of history,
including selling children for debts. Some debts are senior to others, and
the order may be changed by govt action, e.g. the Muni Bankruptcy Act, 1938
or so. Government as tax collector by force represents private creditors
holding public bonds.  Etc.

Mason Gaffney