Messages (Yosemite): Start an audio chat
You can start an audio chat in Messages over FaceTime. You can also audio chat over Messages with AIM, Jabber, Google Talk, or Bonjour.
If you have an iPhone with iOS 8 nearby, you can make phone calls from your Mac.

Start an audio chat
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Send a message to the person you want to audio chat with, letting him or her know that you want to audio chat.
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Click Details, then choose the Audio button
. If Audio isn’t listed, you may need to sign into FaceTime. You can choose which audio input device to use in Sound preferences.
Join an audio chat
When you’re invited to an audio chat, a small window appears notifying you of who invited you.
Click the window, then accept, decline, or reply to the invitation.
If you reply, the audio chat is rejected.
Enable or disable audio chat
When you disable audio chat, you can’t start audio chats, but you can receive audio chat requests. The person you audio chat with can’t hear you, but you can hear him or her.
Choose Video > Audio Chat Enabled.
When audio chat is enabled, there’s a checkmark next to the menu item.
Make a phone call
If you have an iPhone with iOS 8 nearby, you can make phone calls from your Mac using Contacts. Calls are relayed through your iPhone, but you use your Mac microphone and speakers to communicate. Phone calls use cellular minutes. Cellular charges may apply.
In the Messages window, click Details, then click the audio button
.
Your Mac calls the number listed in Contacts for the person you are having a Messages conversation with.