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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We needed a new 18 line telephone system for our purpose built factory. Birmingham Telecom were extremely helpful, knowledgable, and good value. The installation and training went smoothly and we are very happy with the Panasonic system we purchased - it has worked without any problems since the day it was installed and we have been able to fine tune the configuration to make it work just as we want. If your looking for a provider, I thoroughly recommend Birmingham Telecom
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