Red status bar light

The red or orange dot means the microphone is on. Which, if you think about it, is essential to have a phone conversation.

ALL it means is that the microphone is on. Sort of like the “ON AIR” sign outside of radio and tv studios.

In addition to the above correct info—gotta have a microphone active for the iPhone to “listen” to what you say on the call to transmit it to the speaker on the far end of the call, after all—here are most of the other indicators and icons used by iPhone: Status icons and symbols on your iPhone - Apple Support

The microphone-active icon is usually 🟠, though it’ll show as 🔴 on some backgrounds; where the 🟠 icon won’t show up as well.