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.
When was the last time you were placed on-hold and felt like this guy because you had to listen to: -A message recorded (probably by an employee) and added to, over and over again at various levels?

*This does not include phones purchased off the shelf at office supply stores such as Staples and Officemax. For the most part, these phones do not have the Message on Hold capability built-in. Add-ons are available for some models but their functionality can be problematic or inconvenient for the user. The best course of action is to upgrade to either a basic key phone system or one of the new virtual phone systems, for which Holdcom is a key audio production supplier. Contact Us for more information.
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.