John works as a Network Security Administrator for NetPerfect Inc. The manager of the company has told John that the company's phone bill has increased drastically. John suspects that the company's…

John works as a Network Security Administrator for NetPerfect Inc. The manager of the company has told John that the company's phone bill has increased drastically. John suspects that the company's phone system has been cracked by a malicious hacker. Which attack is used by malicious hackers to crack the phone system?