People don’t like going on hold, but you knew that. Funny, considering the early telephone users had to wait to be connected anyway, but now we expect an immediate connection. Think back a couple of decades to the dial-up era. You probably remember the time it took for AOL to connect—and the sounds that went with it. Now, a web page that takes more than a few seconds to load is inexcusable.
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.

A phone line isn’t a state of the art speaker system; its primary function is to make sure the call is connected. Volume changes, frequency, and distortion are all elements to consider when choosing hold music.
Forming first impressions is an important part of building strong relationships with people. Many times, the first contact between a customer and your business is on the phone. It is estimated that 70% of business callers experience some type of hold time and that time spent on hold can be the first impression of your company. This can influence whether your caller chooses to do business with you.