Most business-class phone systems come with a library of pre-recorded on-hold messages or music for you to choose from. They also typically allow you to upload your own, giving you the opportunity to customize your callers’ experience while they wait.
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.

No matter which kind of company you call — big or small — their on-hold music usually sounds like it has been recorded in a tin can. You would think that with modern technology being able to clone sheep and fly humans around faster than the speed of sound, that a creating a business phone system with clear and crisp on-hold music would be easy. But as it turns out, it isn’t so straightforward.
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