This.
For an incoming call, the window should contain a large warning - but also toggling the mic HKS during that window (i.e. after the call becomes incoming and before you answer the call) must work.
For an outgoing call, the Calls app should produce a large warning - and I guess also the Contacts app would have to activate a similar warning when you actually initiate a call.
With the complication that if you are using a Bluetooth headset, that would need to be taken into account (once Bluetooth headset actually works, and bearing in mind that your car could be your headset).