Actually you can. I've done it many times as this photo will show. It is a dual purpose jack and many phones have them but that information is not usually in the manuals... but a quick search on Google/YouTube will show it. For this shot I was using a Movo VXR10 microphone.
The issue I have is that when you do use an external microphone, the speaker is deactivated; on the G7 at least. I suspect the reason is the phone thinks you're using a headset with a microphone, even though it shows an external microphone option in camera settings. That makes no sense, but anyway. I would like to have the speaker functional as needed when I have a microphone plugged in.