This means OnSIP customers can now insert custom music, advertisements, company announcements, self-recorded messages, or any other pertinent audio media, into their on-hold music. Additionally, these custom messages can be scheduled to play at specific times on specific days!
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.

The problem, in my mind, is that every single one of these articles misses the point completely. Even if you implemented a call centre system that detected and automatically played someone’s all-time favourite music while they waited, a facility which is possibly not that far off, most music when played down the phone just sounds terrible. Butchering a song that someone actually likes is unlikely to improve the waiting experience.
There are three parts to a perfect on hold message: the script, the speaking tone and the written tone. And of course, there is also the music, which plays a big part in representing your business brand.