law: 4. Voluntary service in lieu of national service.-
  (1) If a qualified person has been enlisted under any other law for the time being in force, for service in one of the Armed Forces of the Union for a period of not less than four years, he shall perform the service required of the members of that Force in lieu of the national service required under this Act.
  (2) If a qualified person has rendered or is rendering service, other than service in one of the Armed Forces of the Union and such service is declared by the Central Government to be equivalent to national service, the period of such service shall be deemed to be service in lieu of the national service required under this Act and he shall (unless he has ceased to be liable under this Act to be called up for national service), be liable to be called up for national service for such term as will, together with the service completed by him, be equivalent to the term of service for which persons are liable to serve under this Act.
  (3) If any qualified person has been enlisted as a member of-
  (a) the Indian Reserve Forces, regulated under the Indian Reserve Forces Act, 1888 (4 of 1888),
  (b) the Territorial Army constituted under the Territorial Army Act, 1948 (56 of 1948),
  (c) the Air Force Reserve or the Auxiliary Air Force Reserve constituted under the Reserve and Auxiliary Air Forces Act, 1952 ( 62 of 1952),
  (d) the Indian Naval Reserve Forces raised and maintained under the Navy Act, 1957 (62 of 1957), or
  (e) any other Force of the foregoing nature,
  he shall not be called up, so long as he continues to be a member of that Force, to render national service under this Act:
  Provided that he shall, after he has ceased to be a member of such Force, be liable to render national service (unless he has ceased to be liable under this Act to be called up for national service), for such term as will, together with the actual service rendered by him as a member of that Force, be equivalent to the term of service for which persons are liable to serve under this Act.