An anonymous stalker hijacked Heather and Courtney Kuykendall's cell phones to spy on them and threaten them.

"They are listening to us and recording us. We know that because they record us and play it back as a voice mail," said the Kuykendalls.

When Target 11 heard about this national story, we wanted to see how easy it really is to take control of someone's cell phone. It turned out, it's not that hard at all.

"You think your conversations are secure but in reality someone can be listening to you," said Target 11 investigator Rick Earle.

Carnegie Mellon University professor and cell phone expert Priya Narasimhan showed Target 11 just how simple it is to spy on others using a cell phone.

An Internet search turned up several sites offering spyware for cell phones.

For about $300, Narasimhan bought the software and downloaded it onto her cell phones.

"It's not illegal to buy this and download on a phone you own. Once you install it, what this gives you is the ability to monitor that phone remotely from anywhere else. You can get to anything happening on that phone remotely without the person holding the phone ever knowing your doing it," Narasimhan said.

With Target 11 producer Mary Ann Vandevelde on the CMU campus, the professor on the second floor of a nearby building and Target 11 reporter Rick Earle eight miles away back at the station, Target 11 put it to the tests.

When Vandevelde calls Earle, professor Narasimhan's phone alerts her that a call is being made. She logs on to her computer and hears every word.

"How about this story? Can they actually hear what you are saying?" asked Earle, while talking on his cell phone.

"Yes they can," Vandevelde responded via her cell phone.

"You're at CMU, but someone else is listening to us and they can hear every word you're saying," Earle said.

Target 11 discovered it's not only every word, but every text message sent to and from that cell phone.

"Here's the sms message saying hi what's up, and telling me what number it was from and what time it was,"said Narsasimhan.

Target 11 also found out it even works when the phone is turned off.

"I could hijack the microphone and completely hear everything you say," Narasimhan said.

Target 11 put that to the test as well. Vandevelde, holding her cell phone in her hand, asked a student for directions. With the student's permission, professor Narasimhan listened in to what he was saying.

While not illegal to buy and download the spyware on a phone you own, once you start listening without permission you could be breaking the law.

"Certainly it's a violation of someone's privacy," said the professor.

Everyone involved in this story knew their conversations were being listened to.

The best way to make sure someone doesn't use your own phone to spy on you is to never let it out of your sight.

A person needs to have access to your phone in order to download the spyware.

Also, phones with Internet access are more vulnerable to this spyware.

Unfortunately, there aren't many ways to know if you're phone has been hijacked.

If you notice your data usage is way up, that may be a signal that someone's listening to you.

And lastly Narasimhan suggested using anti-virus software to screen your phone, just like you use on a computer.