There are many ways to make voice calls on your iPhone. The representative is the voice call service (carrier call) provided by the mobile phone company. It is an old-fashioned method of calling by using a mobile phone number as a landmark. The number of users of the calling functions of SNS services such as LINE Audio and Facebook Messenger is also increasing rapidly.
And then there's "FaceTime." This is a service limited to Apple products such as the iPhone, but the high-quality sound makes it easy to hear your voice, and since it uses the data communication network, there are no call charges. Using the FaceTime app, you can make a call using the other person's email address or phone number as a mark.
FaceTime can also be switched in the middle of a carrier call. If both parties on the call are iPhone users, the FaceTime button on the phone app keypad (screen during a call) will appear white, and when you tap it, the elapsed time displayed under the other party's name will be displayed as "FaceTime.. ", and the image reflected by the front camera, that is, your face, will be displayed on the iPhone.
At this point, the other party has not agreed to switch to FaceTime. The other person's screen will display "Answer" and "Reject" buttons, tapping the former will switch to FaceTime, tapping the latter will return to the carrier call. And you can't see each other unless you switch to FaceTime.
In other words, tapping the FaceTime button is an invitation to switch to a video call, and it's up to the other party to accept or not. Even if the other party rejects the call, the original carrier call screen will be displayed and the call will continue.
The FaceTime button on the keypad is limited to video calls (video + audio). FaceTime also has a voice-only service "FaceTime Audio", but you can't switch during a carrier call.
