My attachment told that consonant sounds are above 3.5kHz and are not produced in ordinary telephones.

 

Here is an example of what quality you should expect. The original sound is a wav file sampled with 44.1KHz. I applied a 20. order 3.5KHz digital lowpass filter on this signal. And then I resampled it to 8KHz and stored it as a wav file. It is readable but you have to concentrate more to get the words.

 

I can never understand lyrics with a heavy African (American?) accent.
The spectrum analyser on my pc shows the sharpest high frequency cutoff that I have ever seen but the music sounds like it has frequencies well above 3.5kHz.

 

The spectrum analyser on my pc shows the sharpest high frequency cutoff that I have ever seen but the music sounds like it has frequencies well above 3.5kHz.

So you had to check the spectrum