Recent rumor I've heard: safety flashers in the building will disrupt phone communication for hundreds of feet around.
Really? Well, it could make sense. Risetime of arcs can be in tens of nanoseconds. If the discharge circuit on a fire-alarm warning strobe was loop-shaped, it could be emitting very brief microwave pulses of few hundred watts.
Time to experiment. Paint your xenon strobe opaque, so nobody figures it out. Maybe "phone jammers" need no transistors.