Solo guitar and piano music can also be decent choices provided you avoid songs with extremes of high and low volumes and pitch. Most phone systems come with default tracks that, coincidentally, fall squarely into these categories. If you’d like something a bit different then you can find plenty royalty-free tunes available to purchase online (they’re not all that terrible), but remember you'll need to pay for a PRS license if you want to make use of tracks by a recording artist.
Website: https://support.zoom.us/hc/en-us/articles/360028212652-Customizing-audio-greetings-and-hold-music

When JustMyVoice Production records your on-hold message you'll have peace of mind knowing it will be recorded and delivered in the quickest time frame possible; with information that's current, informative (and maybe a little entertaining) about your business.
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.