FaceTime

Thanks for letting us know where you purchased your iPhone, according to Apple, ‘FaceTime Audio is not supported in China mainland.’ So depending on your current location, this could be related to why this is happening. You can read more about this here:

Wireless carrier support and features for iPhone in Asia-Pacific

Take care.

Hey there ffrombaku,

Thanks for reaching out to Apple Support Communities. I understand you’re having issues making audio calls over FaceTime. I’ll be happy to help with this.

To isolate the issue, can you still make normal calls, record audio with Voice Memos, and use Siri without any issues? If not, the issue could be with your microphone. In this instance, this link could help:

If the microphones on your iPhone, iPad, and iPod touch aren't working

FaceTime and FaceTime audio calling may not be available in all regions. Go through the steps in this link to help you identify if you can use FaceTime audio calls in your area, as well as other tips to help resolve the issues you’re reporting:

If you can't make a FaceTime call on your iPhone, iPad, or iPod touch

If the issue continues after taking these steps, you’ll want to reach out to Apple Support using the following link to continue to look into this with you:

Contact Apple Support 

Cheers