Please make sure that Microphone access is turned on.
Swipe down twice and look for a "Mic access" quick tile.
If Mic access is turned off, no apps are allowed to access your microphones, not even the system dialer.
When the Mic access is turned off and you get a call, you need to unlock the phone to turn Mic access on.
Yes, you can accept or reject incoming calls without unlocking your phone, but in order to turn Mic access on you need to unlock it.
My personal recommendation is to keep Mic access on during "work hours" (when you get/expect a lot of incoming calls). Later on, you can see which apps accessed your microphones by checking the Privacy Dashboard.
Hope this helps.