I need an app to make free calls overseas

My brother got a job in London and is going to be away 24 weeks a year.

What's an app that we can use to talk on the phone that only uses data?

I'm in USA.

Have your bro sign up for a Google Voice line before he moves overseas.

Use Hangouts for all calls.

He'll have a US number.

WhatsApp does voice and video call over wi-fi. I’ve used it for years to keep in touch with my American contacts. It’s all free of charge.
WhatsApp works great and is free(needs internet though) . I use it to talk to my sister who lives abroad.

Also if he is going to spend a lot of time in Europe he will probably end up downloading and using WhatsApp anyway
If he's moving to London, it'd be best to get WhatsApp for him since unlike America who seem to have a fear of it there's a lot of people who use it in the UK. So yeah, another vote for WhatsApp.
I've tried a few apps in the past. Whatapp has worked great for me. Typically I use it because we have some cell service deadspots, but still have access to wifi.


One really creepy thing I do is just repeatedly shit on GAF while sarcastically calling it a "top tier gaming forum" so feel free to report me if I keep doing this.
Whatapp or facebook messenger