No null is fine if you don’t know or there’s literally no value. But don’t interpret a null phone number to mean the customer doesn’t have a phone number. You can’t infer anything from that, other than you don’t have it.

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.