The most common codecs are G729 (compressed) and G711 (non-compressed). They are designed to compress human speech, and often cause a loss of audio fidelity — which is worse in some languages than others. On hold music that has been specifically designed for that purpose, to be used with VoIP systems and codecs, can sound great. But it is not easy to come by.
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 experienced Holdcom Script Consultant will review your website and other marketing materials to create an effective audio marketing message.
Using concise and simple words in your scripting helps ensure callers can comprehend what is being said. Although a complete on-hold message can be quite lengthy, it should include concise and to the point single messages. Aim to get each message across in around 35 to 40 words. Remember, what may read great in written word doesn't necessary sound great said out aloud.