You never know who might be listening in on you iPhone conversations — unless you have Signal, in which case the answer is probably "no one." The Signal privacy app has made its way to iOS, making it the first free app that offers end-to-end encryption on Apple phones.
Signal comes from Open WhisperSystems, which privacy buffs may recognize as the company behind the RedPhone and TextSecure apps for Android. Like the company's other projects, Signal is open source, meaning that you can verify the program's security for yourself, or offer your own improvements.
The program itself is extremely easy to use. After a one-time verification code sent via text, users can open Signal and converse with others via their regular contacts lists and phone numbers. Keep in mind, though, that while Signal uses your phone number, it uses Wi-Fi or mobile data rather than voice minutes, so using it away from a Wi-Fi network could get costly.
Also, while Signal encrypts the contents of voice calls, it doesn't hide which telephone numbers are making and receiving the calls. Law-enforcement agencies are often just as interested in who's speaking to whom as in what's being said, so if you want to disguise that aspect, you'll have to combine Signal with apps that provide disposable phone numbers.
Just to make sure that the line is secure, Signal requires each user to speak two random words displayed on the phone screen aloud. This means that, in theory, a line could never be secured if a third party were listening in, as he or she wouldn't know the code words.
Signal is also fully compatible with RedPhone for Android, meaning that Apple and Android users can communicate with one another securely. In a few months, Signal will get support for text messages, and RedPhone and TextSecure will become a single Signal app on Android as well.
Whether anyone is really listening in on your phone calls is a fair question (unless you're a criminal, the government is not interested, and if you're a criminal, the government can legally listen in anyway), but Signal is still just about the cheapest way to keep your iPhone conversations private.