A method (FIG. 3), corresponding call screening unit, and base station (FIG. 2), suitable for detecting cloned communication units (111 or 113), are operable to receive a first response message and a second response message (305); determine whether identification fields (ESNs, MINs) corresponding to the response messages are equivalent (307); and if so, assess whether message contents or message properties corresponding to the response messages are not correlated (311) thus indicating the response messages are from different communication units; and when not correlated, decide that one of the response messages corresponds to a cloned communication unit (313).