Default [Q] Can be smartphones listened by police ? How/When ?

Ok, so i asked myself this question for a long time ... Can be smartphones listened by police or others ?

First of all i know the conversation we do on smartphones can be recorded by the operator ( vodafone, orange, etc ) BUT if the phone is on stend-by they can hear us too ? Is there any Hardware part who do that or a procces ?

So ?
Samsung Galaxy S4 (i9505)