Possible Clues:

• Join up and listen for this
• On European borders, be inclined to augment army
• Call up some women listeners
• Perhaps fifty is in ten in you join up
• Prepare to serve
• Join up (in army)
• Recruit gets silent treatment
• Learner is brought into part of hospital as recruit
• Recruit ordered to be silent
• Learner is entering part of hospital to sign up