Porting a landline number to a secondary Ting eSIM on iPhone with primary AT&T account

This was much harder than I expected it to be. Below is a simplified account of what eventually succeeded. I actually started out by trying to transfer from CenturyLink to Tello. That port failed . It was impossible to determine why it failed, each company pointed me to the other. I did learn that the porting process is surprisingly manual, there's an actual carrier to carrier voice call involved.…