As the name implies, you have to be very close to the radio transmitting in order to get a hit. The signal (from your 'target') has to be significantly stronger than any other radio signals it can receive. How well it works is dependent on how close you are to the target you are trying to find, as well as the strength of other signals from other transmitters. It does not work well on DMR (if you have that upgrade) because the handheld & mobile radios do not transmit continuously. They transmit on one slot for a fraction of a second, then pause while the other slot might be in use from a different transmitted. Close call requires a continuous signal to work properly. That alternating transmission is not continuous, so Close Call does not capture it.
See this Wiki page for more details on how it works:
wiki.radioreference.com