I just tested things with my Mac (running Sequoia 15.7.4) and iPhone (running iOS 18.7.2).
This was with the "iPhone Cellular Calls" feature set up.
Continuity features and requirements for Apple devices – Apple Support
When I enabled iPhone Mirroring, and attempted to place a call from the Phone app in the iPhone Mirroring window, I got the very specific error message "Calls are not available using iPhone Mirroring." When I tried to place the call using the Mac's FaceTime app (the normal way to use the "iPhone Cellular Calls" feature), the call silently failed.
When I called the iPhone, the iPhone rang. Answering on the iPhone ended the iPhone Mirroring session. When I repeated this experiment, I noticed that the Mac also displayed a notification about the incoming call – and let me take it. (Given how specific the "Calls are not available using iPhone Mirroring" message was, the ability to accept calls on the Mac without losing the iPhone Mirroring session may have been an unsupported feature.)
I have not tried turning off "iPhone Cellular Calls" to see if this would allow placing and receiving phone calls using the Phone app in the iPhone Mirroring window.
[EDIT: Just tried that now. Turning off "Allow Calls on Other Devices" didn't allow placing calls from the iPhone app in the iPhone Mirroring window.]