Message On Hold is the most effective marketing tool for promoting your business to prospects, while providing customer service information to your customer base. Variously labeled as telephone on hold messaging, phone hold music, marketing message, telephone on hold message, on hold communication, on hold marketing, or another 50 variations, the programming you play to callers on hold is an essential communication tool. Think about it: who is more captive than a caller on hold?
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.

“Great service on offer from Media Music Now, quick, efficient and simple to use. Quality of audio for an inbound call centre excellent! Will be using again." Chris Hall, GAP Ltd
Suddenly, your optimism regarding the call takes a nose dive as your Customer Service Representative says flatly, “I’ll have to put you on hold while I look into this for you.” Or, worse yet, “Let me transfer you to the right department. You’ve reached the [insert the name of a department that has nothing to do with your needs] department.”