Google Hangouts is what I use for work via wifi/Digicel router. Hangout does not show a caller id when you make a call, but it is quite easy to do wifi calling. Whatsapp is what we end up using when we're working with an international team in Europe, India and China.

Winterstale, I haven't seen Viber anywhere else, but it looks like its the chat app embedded in a lot of games.

Cheers, RickG