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.
Our 249-bed facility is located on the North Well University Hospital campus in Manhasset, Long Island. This close proximity to the hospital allows residents to receive state-of-the-art emergency care when needed.

Website: https://www.tunepocket.com/best-on-hold-music/#:~:text=Here are 3 simple tips to help you,to the lower bandwidth of the phone line.
Music On Hold has decades of experience in the voiceover industry offering professional recordings nationwide with a quick turn around including phone greetings, commercials, overhead music, and voice narration.