My phone is 6 days old, & having the same issue intermittently, which is difficult to pick up until you make or receive a call, & then realise the other party cannot hear you.
Updating to IOS 18.1.1 has made no difference.
Switching off & back on resolves it initially (if I make a call immediately after reset other party can hear me) but it is recurring.
Been on a call with Apple twice (AppleCare & Post Sales) for almost 2 hours - remote diagnostics pick up nothing.
Disappointingly Apple refuses to replace the phone, & expects me to send the phone from New Zealand to Australia!