GSM800 cell phones have their frequencies separated by 45MHz exactly. So when any cell phone tries to make a call - it becomes blocked by itself! The phone talker will hear its own voice.
You can also keep in mind that this cell phone signal jammer can block all cell-based trackers which use your car GPS to track you down and record the data. Even IEDs are probably could be jammed, at least those ones controlled by cell signals.