Download the DIGITS app from the App Store or Google Play Store, and log in using your T-Mobile ID credentials. If you don’t have a T-Mobile ID, the app will allow you to create one. Just remember to use a voice line for creating the T-Mobile ID. Go through the setup process and decide on which line you would like to use in the DIGITS app.
If you don’t see one of the lines that you want to access, continue going through the setup procedure. Ensure that you have the right permissions to use the DIGITS line by going to Settings >> Advanced options >> Line permissions. You will see all the lines that you are permissioned to use and share. Click on the line that you want to access and share it with the T-Mobile ID that needs to use it, including your own. For more information, click here.