 Casual listeners easily heard conversations as plain narrowband FM; eavesdroppers with specialized equipment readily intercepted handset Electronic Serial Numbers (ESN) and Mobile Directory Numbers (MDN or CTN, the Cellular Telephone Number) over the air.The intercepted ESN/MDN pairs would be cloned onto another handset and used in other regions for making calls.Due to widespread fraud, some carriers required a PIN before making calls or used a system of radio fingerprinting to detect the clones.