The point is, once the noise and content are one file, separating them is not an exact science.
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.
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.
Isn’t most conversation below ~3 kHz, while the noise is high-frequency, like tape hiss?
If so, there’s no separation problem.