That's very interesting. How long was it powered off for? A few seconds, or longer?
Did you do anything else between restarting the phone and receiving the incoming call?
What do other people with faulty mic get from powering off the phone and restarting it? Two questions: a: After power off, leave it for a minute, power on, does the mic work? Try this 5-10 times, in case it's an intermittent boot time issue with initialising the mic device. b: After power off, leave it for a minute, power on, then accept an incoming call from another phone (without doing anything else since the restart), does the mic work? The reason for asking is to check if there's an intermittent driver issue with initialising the mic device.