[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.

[Tex-Con-Man]

Cell phones are designed to transmit voice...which means a pretty narrow frequency range. I would guess somewhere between 250HZ on the low end, and around 8-10KHZ on the high end.

So any noise transmitted through a phone will be already be band-passed filtered into the vocal range.

For instance, if Zimmerman slammed his truck door at the exact time he said the word “cold”, it would be difficult to separate the 2 sounds once they become one audio file.