Answer FaceTime video call with only audio

If you already tried (to no avail) ALL the steps provided in this support article:--> Use FaceTime with your iPhone, iPad or iPod touch – Apple Support (AU), I suggest that you make an appointment (*) and have your device inspected at an Apple Store or AASP (Apple Authorized Service Provider) among this list:--> Apple – Location Selector. After checking it out, they'll give you all the available options.

Regards

Giulio

(*) You can do it:

• Online:--> Genius Bar Reservation and Apple Support Options - Apple;
• Contacting a local Apple Store or AASP among the list provided above;
• Using the "Apple Store" app, available and downloadable from AppStore.

FaceTime audio may not be available in all countries --> Group FaceTime and FaceTime Audio are not available in China mainland on iPhone and iPad with cellular.