They have to at least be paid a certain amount. No one really volunteers for the government, union rules.
You remember the stories during the shutdown that Catholic priests who had contracted to the DoD where no longer able to offer communion - even on a volunteer basis - because the gov’t decided it was illegal to volunteer.
The same logic was used on many gov’t employees - they were told not to even attempt to read email, let alone respond, because since they were not getting paid it would be viewed as an act of volunteerism and therefore it was illegal.
Nice to know that when big O is in trouble, the same laws don’t apply to him.