You can use Viber, Viber uses the same method wherein you gonna need use or call to actual mobile numbers for you to be able to call to anyone who uses the same flatform. And it works as long as you have internet connection.
I would recommend Discord. You can chat, send messages and even add lots of different bots so that you and friends can listen to music, play a certain game, or keep track of certain details. There's too many to name them.
You can talk in private conversations or create a private Discord server for all of your friends. It's completely free (with Discord Nitro for extra options, but completely optional/ignorable)!