To set/change the default 2FA method follow the steps below:
Log into the chrome browser with your Willow email account
Click on the Google account icon as shown below. Click on the icon that I put a box around.
Click on Manage your Google Account
Click on Security on the left hand side.
Under How you sign in to Google you should see 2-Step Verification phones.
Expand that choice and you should be able to add your cell phone number.