Hacking into other people's phones is illegal, and constitutes a breach of privacy. This answer will only outline how to hack into a mobile with the owner's permission.
Different Methods of Hacking a Mobile Phone
Mobile phones are more difficult to tap than land line phones, because they use radio waves to transmit information instead of cables. You can listen into a mobile to mobile conversation by using equipment that allows you to tune into certain frequencies. A mobile phone transmits over frequencies ranging from 800 MHz to 2 GHz.
Another way to hijack a mobile call is to make yourself, (the hacker), the local base station. This works by forcing the user to unwittingly communicate through the hacker, thereby allowing the hacker listen to everything.
Hacking for information can be done in two different ways. With the first approach you will need to have access to the phone for a few minutes, because this method requires you to alter a chip within the phone.
This is only possible because of a glitch with the chips used in phones that use Global System for Mobile communications (GSM). However some of the newer phones are not prone to this because they lack GSM support, but even so, around 70% of those can still be hacked using this method.
The other way to hack a mobile would require a laptop with special programs that can decipher the encryption used to protect the mobile phone communication. This method will need the hacker to be in an area with mobile phone activity, and it will also require bluetooth to be activated on both phones.
Jailbreaking Your iPhone
Another popular hack for mobiles is called jailbreaking, which gives you 'root' access to the OS. A successful jailbreak means that the
restraints put in place by Apple will be removed, and all of the
features of the OS will be unlocked.
Jailbreaking brings some benefits: After your iDevice has been jailbroken, you can install Cydia, which is similar to the Apple App Store. From here you can then download other third party applications like games, themes and much more.
Jailbreaking is now legal in most countries, but it will still void your warranty with Apple. However you can easily get round this by restoring your phone to factory settings if you have to return it to Apple. So the only real drawback is that jailbreaking takes up some storage space on your phone. Some people say that you can brick your iDevice (making it useless), but this is not true.