If you have an iPhone running at least iOS 7.0, you should by all means install a new, free app from Open Whisper Systems called Signal:

https://itunes.apple.com/us/app/signal-private-messenger/id874139669

Signal is compatible with RedPhone for Android, and allows you to make free, secure calls to others with Signal or RedPhone installed. As with RedPhone, calls are free because the app uses your WiFi or data connection instead of calling minutes.

In coming months, free secure texting is going to be added to Signal for iOS, and on Android, RedPhone and TextSecure will be merged and re-named Signal.

Even if you're not very concerned about the privacy of your calls, in view of the potential cost savings (especially for international calls) there's really no reason not to use Signal or RedPhone.

Here is a sample screen shot of the Signal for iOS  contacts list. The contacts lists is also able to display a sub-list of all your contacts who also have Signal installed:



This is a screen shot of a call in progress. Note the words "hockey pair." For each call, a new word pair is generated. If both parties to the call see the same word pair, they can be confident that there is no "man in the middle" intercepting the call:



This is a shot of a sample Signal for iOS call log: