All, with some help from Kevin, AI5SM, I was able to get my Digirig working with my KX2 and my iPhone and iPad using iFTx. For those who might be interested, here are some detailed notes:
This setup uses VOX and not PTT.
Connect Digirig to iPhone/iPad using USB-C (or USB-C to Lightning for older phones/ipads) cable. From the Digirig, connect the green lead to the Phones connector, and the red lead to the Mic connector on the KX2/KX3.
iPhone/iPad Settings:
Settings → Sounds & Haptics → Headphone Safety:
USB Audio Accessories → Forget All USB Audio Accessories
Connect Digirig to USB-C or Lightning on iPhone/iPad
Are you connecting a pair of headphones? Select Headphones
Elecraft KX2/KX3 Settings:
Menu items:
Mic Btn: Off
Mic Bias: Off
Vox Md: On
Vox Gain: 10
Filter:
Adjust the filter bandwidth to 3.00
Data Mode:
Ensure Data A mode is enabled