USA Business Telephone Today conducted a survey of 30,000 callers to study the effects of hold music, or the lack of it, for a single minute. More than half the callers who were put on hold without any auditory stimulation hung up. The silence affected the rest of the group’s perception of time: More than 27 percent thought their wait time exceeded five minutes, and 18 percent thought it exceeded three minutes. Only 3 percent judged their wait time to be close to the actual one-minute timeframe. All of the callers in the study had listened to dead air for only 60 seconds.
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.

Website: https://www.beatsuite.com/royalty-free-music/royalty-free-on-hold-music
Free Music On Hold. Here you can download some free music files to test out if your hold equipment is compatible with our music files. These samples are only a short version of each track, you can buy the full length files from the genre list on the left.