Nowadays, may people are using cell phone spy software, which are installed on the person's phone you want to spy on and then you can access almost all and everything from their phone. With this software installed on their phone, you can actually eavesdrop on an on going conversation through that phone. You can access the text messages and E-mails received. Other than this, you can also track the person. You will be able to know the location of the person. This software also enables the phone to be used as a bug. It can also operate when the person is not on call and at that time it can still work as an voice transmitter.
I find all these techie things fascinating. It feels to be part of some bond movie, but no it's not and is available.