On-Hold Messaging has proven benefits when it comes to phone call retention rates and in helping to upsell new products and services. Fewer Hangups Giving your callers something informative or entertaining to listen to keeps them on the line and avoids the dreaded hangs ups - which may result in the loss of a sale.
So modern technology, even with the most advanced VoIP business phone systems, has not improved the plight of the on hold song. If you are interested in learning more, check out Tom Scott's explanation at the Milton Keynes Museum, in their Connected Earth gallery.

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.
Your audio image is important. Phone contacts are often the first impression people have of your business. If they must hold on the phone, this is your chance to impress them with relevant greetings and telephone hold messages. Callers can be existing clients, future clients, potential customers, and they are all important. A professional voicemail recording boosts your credibility, and encourages prospects and potential connections to leave a voicemail when you can't answer the phon.