The best hold music has few instruments and preferably only one. Simple music handles the change in quality better than a full orchestra arrangement. The latter gets distorted, particularly if it’s too loud for the phone to handle and clipping happens. No one wants to hear a dirge-like cacophony masquerading as “Pachelbel’s Canon in D.”
So what can we do to avoid inflicting further musical misery on our own customers while they’re patiently sitting on hold? We could just go back to playing a ringing sound, or an intermittent beep just to let people know they’re still connected, but that would be awfully dull. The best compromise is relatively simple electronic music that doesn't have too much going on as the ideal choice for hold music as it requires far less refined sound quality to get a decent rendition down a telephone connection.

Hold music gained mainstream popularity in the early aughts. As cell phone customization became more of a focus, ringback tones gained popularity. People carefully chose the song to play in lieu of ringtones for incoming calls. It was a chance to control the call experience and showcase your personality before you even said hello. By the time the first US national carrier offered ringback tones in 2004, it was a multibillion-dollar industry. Hold music keeps your callers distracted from the wait. Why Use Hold Music at All?
Website: https://www.alternativetechs.com/wp-content/documents/USER.SV8100_with_SV8100_12-and_24-button_phones.line_keys.pdf