Article 5 isn't voluntary.
NATO’s Article 5 is consistent with Article 51 of the United Nations Charter, which recognises that a state that is the victim of an armed attack has the inherent right to individual or collective self-defence, and may request others to come to its assistance. Within the NATO context, Article 5 translates this right of self-defence into a mutual assistance obligation.
https://www.nato.int/en/what-we-do/introduction-to-nato/collective-defence-and-article-5