facetime

See this about Group FaceTime. Use Group FaceTime on your iPhone, iPad, and iPod touch - Apple Support

All devices must meet the requirements to be used, otherwise they can only use audio in the group mode, not video.

Is your device running iOS 12.1.4 or later? The bug fix that dealt with the issue that turned off group calls was fixed in 12.1.4.

Use Group FaceTime on your iPhone, iPad, and iPod touch - Apple Support