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.
As part of our strong customer service commitment and our lifelong competitive strategy, we have established our Quality Assurance Lab program. We have modern equipment to virtually test any durability, precision and performance. One of our excellent representatives will be happy to tell you more about it in a moment.

Cornell's "I'm On Hold" follows some of the industry standards, particularly around frequencies and compression, which is one of the reasons he wrote it with a country-ish melody, wanting something simple but catchy, with only a single instrument and vocals. But in other ways, "I'm On Hold" totally upends the genre by breaking the cardinal rule — that hold music shouldn't call attention to itself.
Of the 10,000 callers who heard hold music during their minute wait, only 13 percent dropped off, and more than half guessed their wait time was less than a minute. To take it a step further, the study's last group heard a mix of music and messages while on hold for one minute. Two percent hung up. Another 81 percent thought their wait time was less than a minute.