Viber began life as a voice calling app. You used to be able to call people on the service along with regular phone calls. It has since evolved into a full-fledged messaging service. You can still make phone calls like you could before (for a fee). You can also text chat, video call, and more. It also features encryption on voice, text, and video calls between Viber users. There are a few extra fun features like hidden chats as well. It's a little bit heavier, similar to Facebook Messenger. However, it's otherwise not bad. The app isn't overly popular in the United States, but it's quite big Internationally. The in-app purchases are for things like stickers and other personalization items.
Signal Private Messenger is one of the most popular privacy chat apps. It features end-to-end encryption for all messages, voice calls, and video chats between two Signal users. It focuses a lot on individual chats. There are group chat features, but it's mostly for personal use. This is one of the biggest differences between it and rival privacy chat app Telegram. Video calls worked fine in our testing so we have no trouble recommending them. It's a neat way to get some encrypted chats as well. The app is also completely free and open source. You can't really go wrong with it, but there are better video chat apps if you need group video chats.