You need to enroll each number individualy in digits beta. I believe you will also need a T-Mobile ID for each number or you can link the line to your current tmobile ID.
Had the same exact issue... Did some reading.. Found out you go to this website: https://mydigits.t-mobile.com/consumer
Sign in and grant permission to yourself to use the line. I had my line and 2 free lines from black Friday that I already set up on my GS7E. When I switched to a V20 (got it half off during the one day sale in Feb) I also signed up for the free line (love my 3 free lines) and just now got around to reading up and adding it. Once added it'll be available for use. Good luck and have fun!
This thread was the product of our fantastic collaborators during DIGITS Beta! As many terms have changed and bugs have been exterminated since the Beta launched, the information here may no longer be accurate. If you still have a question, please search our active threads or ask a new question in the DIGITS space!