It's not really a good time for security and privacy. Every attempt at securing communications is being checkmated by folks from NSA and FBI and whatever other government agency. But that isn't stopping developers from figuring out secure protocols and communication apps.
Signal is a new, open-source iPhone app that promises secure calls on your iPhone. With Signal installed, you and your friend can call each other without your call being eavesdropped, snooped or deciphered.
Signal is built by Open Whisper Systems, a company that builds open source security software for mobile devices. Their latest offering, Signal, is an app much like Whatsapp/Viber. You can only make calls to other users who have Signal installed on their iPhones. But what makes it Signal different is that it provides a secure channel of communication.
When you make a call through Signal, the call goes through but the caller (you) and the receiver (your friend) have to make sure that they are seeing the same id number on their screens. This is Signal's way of letting you know that your connection is secure. If the id/numbers you see are different from that of on your friend's phone, the connection is not secure (someone else is listening in.)
The code beneath Signal uses what is called ZRTP, a tested, secure voice protocol.
Signal is being built to work not just for voice calls but also for text messages. At the moment though, the iPhone app lets you make secure calls.
When you install Signal, it imports all your contacts (with your permission of course). All contacts that have Signal installed already are tagged under favorites. If you make a call to someone who doesn't have Signal (or RedPhone, the Android app) installed, they will receive a text from you, inviting them to install Signal.
Signal is iPhone-only but it ties up well with RedPhone for Android (which is Open Whisper System's app, by the way). The devs are also planning to integrate text to Signal.