What is the ruling on wiretapping phone calls?

It is impermissible to eavesdrop on people's calls and conversations, as it falls under spying, which is prohibited by the Almighty's saying: {O you who have believed, avoid much [negative] assumption. Indeed, some assumption is sin. And do not spy} (49:12). Among the forbidden forms of spying is a person listening to the conversation of a people while they dislike it. A severe warning has been issued for those who do this, as the Prophet, peace and blessings be upon him, said: "Whoever listens to the conversation of a people while they dislike it, molten lead will be poured into his ears on the Day of Resurrection." This includes eavesdropping on people's phones and calls, as it is customary for people not to want anyone to know about their conversations. Thus, the eavesdropper, in this sense, falls under the mentioned warning.