Yeah sure you can.
If its DIGITS app in Android:
Go to DIGITS app -> Click the gear icon on top left corner -> Choose a line -> Click on Ringtones and choose the one you like
If its DIGITS app in iOS:
Go to the DIGITS app -> Click on the Gear icon -> Click on the DIGITS number -> Click on Ringtones. You will see a list of 9 ringtones choose from.
Please let me know you can find it. Keep me tagged.
On the LG V20, changing this setting does nothing as digits calls are routed through the phone's default phone app. Thus your regular line and any other digits lines will all share the same ringtone.