How to change the frequency of the Hourly Time Signal on my Casio digital watch?

I have a Casio watch in which the Hourly Time Signal (HTS) announces the stroke of every hour with a beep sound. But in the watch's manual, it is quoted as the HTS "causes the watch to beep twice every hour on the hour". I sure don't know what 'twice every hour' means because my watch beeps only once every hour. But I want to change this frequency and make the HTS beep maybe twice or four times every hour at equal intervals. The HTS setting menu on the watch has a ':00', which probably means the setting is kept at every 60 minutes. But I'm unable to change this to :15 or :30 inorder to increase the frequency of the HTS. Is it possible to change this frequency? What does the manual mean by "twice every hour on the hour"?