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.
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The decision to update your on-hold message depends largely on the content you are sharing. If you are sharing content that includes short-term promotions, this information will need frequent updating so consumers are always aware of the status of any offers. However, if you’re only sharing general information about your standard products, your operating hours and location, then the decision to update the message can be more reliant on metrics.
No one likes waiting around on an endlessly ringing phone line for someone to pick up the telephone and get them the information they’re looking for. That’s why having an on hold message or friendly musical greeting can be a game-changer in the world of client communication.