Collocations for voluntary - adj.

be
remain

completely, entirely, purely, strictly
The organization is run on a purely voluntary basis.

largely

Voluntary is used with these nouns:
act, agency, agreement, arrangement, association, basis, body, capacity, care, consent, contribution, departure, donation, effort, euthanasia, evacuation, exile, guideline, help, helper, liquidation, manslaughter, organization, participation, playgroup, redundancy, regulation, repatriation, restraint, retirement, scheme, sector, service, undertaking, vaccination, withdrawal, work, worker