I'm not sure I agree.

If I have a column for the ID of the customer's current active subscription, and that column is NULL, it seems perfectly fine to interpret that the customer has no active subscription.

That is a valid inference. You don't need a separate has_active_subscription field.

On the other hand, your phone number example is just common sense. The database doesn't represent the external world. The database just knows the customer didn't provide a phone number.