The police were investigated for alleged illegal phone-tapping.

phone-tapping 是由 phone（电话）和 tapping（窃听）组成的复合名词。phone 指"电话"，tapping 来自动词 tap（轻敲、窃听），这里指"窃听"。 phone-tapping 指"电话窃听"，即未经允许监听他人电话通话的行为。