For those still using analogue or digital phone systems, voice on ISDN circuits has a bandwidth of around 4000Hz. The audio from an analogue phone is sampled at 8000Hz, and because of the nyquist theorem, must be low-pass filtered to 4000Hz. So any frequencies in the audio beyond 4000Hz are lost. Interestingly, the human ear and mind are good at recognising human speech, so this is considered "toll-quality" voice, i.e. good enough to pay for. This is the reason why certain vocalised letters are difficult or impossible to tell apart on the phone. For example, the differences between B, P, T, C, etc. are almost entirely above 4000Hz.
Suddenly, your optimism regarding the call takes a nose dive as your Customer Service Representative says flatly, “I’ll have to put you on hold while I look into this for you.” Or, worse yet, “Let me transfer you to the right department. You’ve reached the [insert the name of a department that has nothing to do with your needs] department.”

-Callers stay on the line up to 25% longer when provided with On Hold Messaging versus “dead air” or “background music,” and up to 17% longer than radio.
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