Tired of low quality cell phone calls? With FaceTime, you can make high-resolution calls if you use an iPhone, iPad, iPod Touch or Mac. See how you can do it.

How To Make A FaceTime Audio Call On iPhone And iPad
To make a FaceTime Audio call on iPhone, iPad, or iPod Touch, first make sure FaceTime is turned on on your device. To do this, launch Settings and then press "FaceTime". In FaceTime settings, press the switch next to “FaceTime” to turn it on.

Then launch the FaceTime app. On the FaceTime main screen, press the plus button (“+”).

In the "To:" field, enter the name or number of the person you are trying to call. Then press "Audio".

The call will be made. During the call, you can switch to a FaceTime video call using the “FaceTime” button or you can disconnect by pressing the red telephone receiver icon.

You can also perform a FaceTime voice call going in the "Contacts" application (or in the "Contacts" section of the "Phone" application on the iPhone). Tap the person's name in your contact list.
If the contact supports FaceTime calls, you will see a “FaceTime” entry below it Telephone Number their. Tap the blue phone icon in the “FaceTime” section to make a FaceTime Audio call.

Note that if the other person does not have a registered FaceTime account via their phone number or email, this option will not appear on their Contact Sheet.
How To Make A FaceTime Audio Call On Mac
Making a FaceTime Audio call on a Mac is easy. First, launch the FaceTime application. In the text box labeled "Enter a name, email, or number", enter the name or number of the person you want to call. If recognized as a FaceTime enabled contact, the following buttons will turn green. Press the "Audio" button.

While the call is in progress, you will see a small one pop-up screen in the corner of your screen. Press the "Video" button if you want to switch to a FaceTime video call. To terminate the call, press the "End" button.

One of the great things about FaceTime is that it works between all major Apple devices, so you can use your Mac to make calls iPhone, iPad and iPod Touches. With a clever trick, you can even call one Apple TV.
Source of information: howtogeek.com