Most famous historical reference:
中国人民志愿军, the "Chinese People’s Volunteer Army" that fought in the Korean War (1950-1953) - though that uses
军 (army), not
兵 (soldier). In the modern PLA,
志愿兵 contrasts with
义务兵 (conscript): volunteers serve longer and become NCOs, while conscripts serve a fixed two-year term.