So, given that hold music is such a powerful potential source of frustration, why do companies continue to get it wrong? Reading the articles that are out there, there seems to be a lot of speculation regarding the best choice of music to placate or even entertain customers while they wait in your queue. Some common suggestions include “Hanging On The Telephone” by Blondie, or Lionel Richie’s “Hello, is it me you’re looking for?”. Or how about looking up the customer’s record in your call centre CRM, grabbing their date of birth, and playing something based on their age range - pop for the under 25s and presumably something fusty for the oldies?
We generally recommend that clients invest in a system capable of delivering a custom messaging on hold feature. Callers and clients appreciate the improved service levels and the additional free advertising can help turn your phone system into another revenue stream for your business.

We don’t want that happening to our callers. Adding a little background music or making sure the audio file sounds like a recording will help prevent this.
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.