If you have multiple phone lines, you can make calls with the phone line
you want using the Priority Calling feature. With Priority Calling, you can assign a
phone line or app to use when you make calls.
In the Phone app, swipe down
from the top of the screen.
Tap > Priority Calling.
Set the Use Priority
Calling switch to On.
To choose your phone line when you make a call, on the Calls screen, tap the
Select Line drop-down list. To use a specific phone line, tap
a phone line. To automatically use your mobile line when you're connected to a mobile
network, tap Use the Priority Line.