Do you need your own iphone, or can you use a friend's iphone to fix the settings once?

It probably should be your own device; the audiogram ends up in the iPhone's health app. It can probably be done by someone else if they don't care and don't need to apply a different audiogram for themselves.

That is some nice integration. Having one’s own up to date audiogram stored in a health record and available to doctors and devices.

I was able to use my partner's iPhone to configure some settings.

It's frustrating that the settings can't be changed on Android, and macOS seems to have a subset of settings compared to iOS.