[Tex-Con-Man]

In a recording, if you use band pass filters, you will remove everything within that frequency range. That may include noise AND content. If the noise is within the vocal range, the only way to reduce it's effect is flipping the phase on the noise and playing it with the original. But this has limited success as well. Unless the noise exactly matches the noise underlying the content, it can only reduce it.

The point is, once the noise and content are one file, separating them is not an exact science.

[rolling_stone]

http://www.paranormalghost.com/evp_frequency_ranges.htm

the audio frequency ability of the cell phone is controlling, was it a wideband cell phone ? What equipment was used to record the calls, what frequency response-likely wider than the cell phone.

[avacado]

I’d imagine that cell phone interference is at much higher frequency range than the human voice. It was prpbably quite easy for them to remove it.

[canuck_conservative]

Isn’t most conversation below ~3 kHz, while the noise is high-frequency, like tape hiss?

If so, there’s no separation problem.