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
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.
I've not found any shops locally that take these for repair. After looking at the pictures for the model 11MG0811 it appears that access to the inner workings is similar to an electronic toothbrush that uses a pinch/lock approach which can be quite a bit tougher.