In addition to being able to communicate directly with your contacts, you can also leave them a voice mail for when they are available.
Users can also create open frequencies. Everyone can connect to them to listen, but you'll have to add people as contacts so they can talk as well.
Zello is an application that allows you to communicate with your friends on a free basis. In addition, thanks to this version, you won't even need a smartphone to be able to communicate with them.