Can I use an external microphone on my iPhone 7 for speakerphone calls? I have an external microphone (Shure MV5) which I can connect to my iPhone 7 via a lightning connector.

I know I can use this external microphone to record audio on my iPhone.

But is there any way I can make plain old phone calls from my iPhone, in speaker phone mode, using the external microphone?
No, you can't, because Apple doesn't not allow the choice of the sources. Otherwise, with an old iPhone with the 3.5mm jack, you can cover the last ring to avoid the microphone contact. 

Apple source