My iPhone is plugged into my Mac. I have a nice USB mic on the Mac I'd like to use for calls. I'm used to using Rogue Amoeba's software (e.g. Airfoil) so I know it's possible to pipe the audio around. How do I use the Mac's USB mic for my iPhone?
You should be able to use the FaceTime app on your Mac to place calls via your iPhone.
To set this up it should be as simple as signing into the same Wi-Fi on both devices (Mac & iPhone) and the same FaceTime account on both devices.
From here you can make calls by just typing the number into this box in FaceTime:
As for using the microphone, you can configure which devices to use for speaker, microphone and webcam in the "Video" menu...
Finally once you've linked this all up inbound calls should show up as a notification you can answer on the Mac. If you prefer to answer on iPhone you can do this and then on the iPhone call app you can click the "Audio" option to route the call to your Mac where the FaceTime app will take over...
EDIT: Evidence that FaceTime can make calls from the Mac via an iPhone: