When you tap on someone who is offline, you will see the menu below. The first two options will allow you to call them, either via FaceTime or using their mobile number. Both of these features use Apple's default apps you are familiar with.
If (and only if) you use this method for contacting someone in your Cocoon, an ambient update will be posted when the call is over. Again, this will only appear if you use Cocoon to generate the call.