When terrorists rig their cell phones to detonate explosive devices, they can use the phone's radio function as a remote control. Terrorists can also use a cell phone's vibration function as the trigger. Using a simple, cheap phone, it is easy to rig a cell phone to detonate homemade bombs.
The FBI recently sent a weekly bulletin to 18,000 state and local police officers warning them that terrorists have used modified cell phones to ring and vibrate devices that triggered bombs. To prevent such incidents, the FBI is urging local police to take several precautions.
It is possible to purchase equipment that can jam cellular signals, but such devices have serious drawbacks. Modern cellular networks rely on what is called frequency-hopping, moving their signals across different frequencies to avoid congestion. Attempts to jam a single frequency can cause other signals to jam as well. The result is a massive amount of electromagnetic "white noise" that interferes with all signals, including 9-1-1.
Moreover, the technology isn't very effective. Most improvised explosive devices (IEDs) aren't triggered by a cellular signal; they are triggered by a battery power source, a wire, a fuse and sometimes a commercial explosive main charge simulated with small wooden pieces or even just dynamite.
Jamming equipment can impede half of all cell phone bombs, but it cannot prevent them from being triggered by the alarm function. Federal law currently allows only federal law enforcement agents to use such equipment, but local police fear that waiting for the Feds is too long a response time to potential terrorist threats and suspected plots.