Verbal Content – generally, you should use 100 words to make up 1 minute of content produced with background music. In other words, one to two paragraphs per minute. The topic can range from product promotions to general company information.
Our blog post - how to choose the best voice for on-hold marketing has some crucial information you should consider before selecting a voice-over talent for your on-hold messages.

When it comes to writing comedy, you need to have people around you. There should be people around you that you can bounce ideas off of. Otherwise, you’ll just be telling yourself jokes and slowly go insane as you force yourself to laugh at your own jokes.
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.