Internet telephony is a type of communications technology that allows voice calls and other telephony services like fax, SMS and other voice-messaging applications to be transmitted using the Internet as a connection medium. You can use apps such as, WhatsApp, Facebook Messenger, Viber, Snap Chat, etc. to send voice messages and calls as long as you are connected to the internet.
On Facebook messenger, all you have to do is create a chat room with the person you want to send a voice message/call to and press the call button on the top of the app and your call will be made. With messenger, you can change your voice call to a video call