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.
If you need a more advanced configuration, you can set up call menu options with our Virtual Receptionist feature, for no additional charge. Call menu options enable you to play helpful messages like hours and location, transfer callers to voicemail or to a live receptionist, route callers to specific team members (customer service, sales, etc). Our wizard will guide you through the set-up process. It couldn’t be easier.

So while this loss of quality is fine for regular conversations where it's just people speaking, if you're planning on piping music down the telephone line, you need to remember that the audio quality will, at best, be around 24 times worse than what you'd hear on a CD.
It doesn't matter how great your customer experience in store or with an employee is, if you neglect the on hold experience, you're neglecting the customer. Not sold? Check out Number 5, below: