Of course, the best way according to experts is to turn off your phone and remove the battery... But what if you can't avoid it? One solution is to do a factory reset, which in most cases will erase the software that's spying on you.
Another solution is to update the operating system. Upgrading or reinstalling your phone's operating system will have a similar effect to a factory reset, but this method will erase data and applications, so it's a good idea to back up your files.
How can you detect if you are being followed:
1. Turn off the device
2. Notice if there is anything unusual on the screen…
3. Check if the battery is warm. This may mean that the phone is not actually turned off but is secretly running some program
4. Turn the phone back on
5. Listen for unusual sounds when making a call.
Source: tro-ma-ktiko.blogspot.gr
