Mumble is a free conference call software. With Mumble, you can call your friends.
The data traffic between client and server is fully encrypted and therefore are also you calls.
By creating a channel, you can call different people at once. you can set a password for these channels, so only authorized people can attend the conversation.
You can find out Mumble server here talk.systemli.org
Official Mumble clients exist for Windows, Linux and Mac OS X. For Android and iOS are a couple of alternative Clients (e.g. Plumble) available. Download mumble client for your operating system.