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.
As you begin your search for an on hold messages service provider, you’ll find many companies advertising that they can do the job for you. But, you don’t want just any company, nor do you want the same ho-hum, boring hold-music heard at most businesses, because boring is something your business can’t afford to be. You want a provider that adds real value to your bottom line, you want an on hold company that’s responsive to your needs, has a proven track record, and finally, you want a provider that makes the process easy, so you don’t have to!

4) An experienced and reputable on-hold messaging provider should also provide you with different music styles that will fit the image of your brand.
Website: https://www.radiocreative.com/fun-phone-messages-on-hold-entertaining-and-creative/