Confirm. Same issue on my side. I'm quite frustrated about this. I was switching from 13 pro which had excelent sound call quality.
It's not only about volume that is considerable low, but it's also about the bass and the voice freq spectrum that is given by the earpiece.
Optimistically speaking: might be an iOS 18 issue.
Pessimistically speaking: HW issue.
Thinking about returning the 16 pro.