People you call must download the app in order to receive free calls and the chat application. We won't be accessing the Internet or email unless we can connect to Wifi and will be turning off roaming on our phones.
Making/receiving calls using the Viber app is only "free" if you are obtaining your data signal in a free wifi area (for example, if you hotel offers free wifi). So yes, be sure to be in wifi and turn off roaming or switch to "airplane mode" to prevent accidentally switching from wifi to cell data, for instance, if you're on a Viber call while in your hotel lobby where there is free wifi and you continue chatting as you walk out the lobby and down the street.
I've used Viber for making international calls, but in my case, I was on the receiving end here back in the states and the person I was talking with was overseas. I didn't experience any problems with the connection. It sounded clear like any other call.
I have never used VIber. I have used Google Hangouts to make free calls to or within the US. The advantage of Google Hangouts is that the other person doesn't need to have Google Hangouts; you can even call landline phones from your smart phone. The disadvantage of Google Hangouts: calls to numbers outside the US aren't free, so any call to a number in the country you are visiting wont' be free (but still pretty cheap - you have to buy credit). Then again, to do that with Viber, the other person has to have it, too.
"or switch to "airplane mode" to prevent accidentally switching from wifi to cell data"
If you switch to "Airplane mode", that will disable all transceiver functions including Wi-Fi. A simpler solution is simply to switch off cellular data roaming.
Another point to clarify......
"People you call must download the app in order to receive free calls"
That's not the full story. According to the Viber website, users can call non-Viber mobile or landline numbers at low rates with Viber Out. I haven't checked the rates recently but as I recall they're very cheap (although they won't be "free").
I prefer to go into airplane mode and then turn back on wifi - easy to do - so that I don't get unwanted phone calls at $1 per minute.
I have had great luck with viber, both receiving calls from my kids overseas and making calls for work when I was there. I didn't use viber out though. Everyone I spoke to or texted was willing to load viber on their cell phones.
I prefer WhatsApp instead of Viber for now. It is free and I feel very convenient with calling my Dad who is in Abroad.