Why did Frank Sinatra do this?

After 1963 , Frank Sinatra always carried a roll of dimes with him . What was the reason?

Frank Sinatra's Son , Frank Sinatra Jr. was kidnapped on the 8th of December 1963. The kidnappers would talk to Sinatra only when they were called form a public pay phone. He always a carried a roll of dimes during that time and it became a lifetime habit for him