From what I've read so far, it seems like VIDEO FaceTime will be free for both parties, other than:
- If the party is using WiFi, whatever they have to pay their ISP for internet access. This is usually a flat rate, though some people have data caps.
- If the party is using cellular service (e.g. 4G), whatever they have to pay their carrier for internet access. This may be "unlimited" or charged per GB.
If you change to audio-only then it may become a regular phone call for both parties, which will typically be free within the same country but extremely expensive between two different countries.
Is the above correct? I want to FaceTime my friend in China but I was worried about paying a huge rate. However if the above is correct then we should be OK as long as we are using video. See The Telegraph's Why did my FaceTime calls to Switzerland cost £144? suggests downgrading to audio can hurt, but I am not sure if the article is accurate.