Guardlock, which offers military-grade AES-256 encryption. Lifetime subscriptions are now $99 through TechnoBuffalo Deals.
The Guardlock app, for iOS and Android, lets you dial anyone else with the app installed. You do not actually need a SIM card to use the service, so you are essentially untraceable. Your calls are encrypted end-to-end (ZRTP protocol), with a new key for each call. For added security, Guardlock does not even keep de-encryption keys, and there is no call logging. Folks you dial regularly can be saved to your address book under a nickname, and the app checks the security of each new call you make. Best of all, this service gives you unlimited talktime, with no roaming charges. It adds up to a pretty impressive feature set.
You can subscribe to Guardlock now for the one-time price of $99 to get lifetime service, with updates included.