DISCOVERY: It is actually a defective headphone jack and not the microphone. I got the idea from another post. After a couple of tests to confirm behavior, I cut the end off an old cable and plugged only the dead/dummy end into the phone. The phone now works like normal for both regular and speakerphone calls. I can only guess that the phone's input jack has somehow become damaged. Unfortunate that for now I have to have this tip/end/blank plugged into the phone but it works.