Our previous phone system would provide alerts when a caller was on hold. For example you are on a call and your second line rings. You place the first caller on hold and start talking to the second caller. Every thity seconds you would hear a beep-beep-beep reminding you that you have someone on hold. If you put someone on hold and hang up your handset then the phone would ring with a different ringtone every thirty seconds to remind you.
We just had one of our customer service reps forget they put someone on hold and finally remembered and picked them up after 15 minutes. Sometimes the visual blinking of the line is not enough.
Dave Cline