A cell phone can be illegally tapped by downloading a specific piece of software that can be created by a variety of foreign "spyware" companies. This software is not a physical piece of equipment, but is a program that can be downloaded onto a phone against the will of the person who uses the phone.
The software that is needed to tap a cell phone can take a large amount of time to be downloaded onto the phone, but once it is on the phone, it is virtually undetectable. The person who has placed the software onto the phone has the option of connecting the phone to a secondary phone line. The secondary line will then receive messages whenever the cell phone makes a call, receives a call, sends a text message or receives a text message. The majority of the software programs that are available also give the secondary line the option to record the information obtained form the tapped cell phone.
As with any other tapping device that is available, the tapping software for cell phones is illegal. It is created and sold from foreign countries to skirt the laws of the United States, but people who are found to be using the software can face strict penalties, heavy fines and even jail time.