Texting app between different countries?

So I'm going to Europe for 3 weeks with a group of students that I don't know. I need an app for my iPhone that lets me texts my friends and family in america for free. Also it can't be something like kik that has to be on a smart phone. I need something that people can receive on regular phones as well.
so.
-has to be free
-has to be able to send messages between smart phones and regular phones
-and has to send them from europe to the united states

Thank y'all!

Viber is the best one

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