التنصت is the word for wiretapping, listening in on someone or something -
a surreptitious listening device in a mobile phone could be called an "ear" أذن for listening in on, or 'spying'. it called also be called an ear plug - سماعة
or possibly سدادة أذن (though this alternative does not imply transmitting any sound or listening/spying)

سماعة مخفية داخل الأذن
سماعة غير مرئيّة