The file is currently packed away, but it was on a form that recorded family/education/work history, aptitude, etc. I've seen 2 or 3 variants of that form and think they had different purposes. Some seem to be from an interview shortly after enlistment, others from after some training was completed. 'It was simply a note the he 'received first call'. It's not a specific box on the form.
Volunteer is a slightly ambiguous term. In the context of the Army in this period, it really means volunteered for active service - could be sent anywhere the Army required, particularly overseas. After 1939/40, lots of guys put off that decision until they were notified that they were to be called up for service.