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The Skype app will have you making calls around the globe with no fuss, and without picking up the bill for high roaming charge.
It's a free app that uses a Wi-Fi connection to make calls to anyone with a Skype username that you are connected with. In a call, you can swipe from the video call to access instant messaging, or view their profile.
If you want to call or text a number outside of Skype, then you can join a subscription service to make cheaper international calls.
Be careful of using Skype on any pay-as-you-go Internet connection, as it does (naturally) hog a vast amount of bandwidth.
I've used this app many times around European jaunts, and it is a lifesaver (when my carrier chooses not to block my smartphone by accident). It's free, updated regulary and displays the transport networks of over 400 cities across the world.
You don't need a network connection once the cities are installed, but if there are transport disruptions, updates can be found by turning on your Wi-Fi.
A free but advert-supported app, this service allows you to send text messages in real-time as a group chat. In a nutshell, it is a chatroom, no matter what phone your friends use. A new feature is the ability to add voice message -- although I found the app useful based on its simplicity.