Actually, they can. Phones using the analogue and 800 MHz digital bands can interfere with DME and ILS receivers. Interference with ILS *shouldn't* cause a crash but can cause enough problems on the approach that the crew would have to go around.
As far as I'm concerned, the myth of cellphones interfering with aircraft avionics was thoroughly debunked on Mythbusters. This confirmed what I--as a former Naval Avionics Technician--already suspected.