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.
some newer cell phones are wideband not voiceband and if so noise outside voice frequencies could be filtered out...it would be interesting to know what type phone they both had as well if they had gps...and if the pd call center (non 911 could track the gps)
http://en.wikipedia.org/wiki/Wideband_audio