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.
Whether it’s music, marketing messages, or a combination of both, the recording should be long enough so it doesn’t loop back to where they started when they were first put on hold. If you don’t typically have long wait times, this should be no problem. If calls get stacked up sometimes, you’ll want to make sure that the recording is at least as long as your typical wait time on a busy day.

When you place a call on hold on your cell phone how do you get it to play message and music? Can you help us by answering one of these related questions?
To serve you better, we will be moving to a new, larger office & showroom on March 1st. Our new address will be 32C Spruce Street in Oakland, New Jersey. And our new phone number will be 2 zero 1- 3 3 7- 6 2 2 2. We look forward to providing you with the same great service from our new facility.