How to valorize Volunteering work in a NPO
TLDR

If you did significant, relevant work for this organization, I would remove the word volunteer and treat it as any other experience. If however you did occasional work or work you didn't learn from, I would leave it as you did above.

From personal experience

I volunteer to build a certain NPO's data infrastructure from the ground up. I find this work mentally stimulating, educational, relevant, and like any other FT employment. Thus, I'll include this in the experience section of my resume and not write volunteer next to it on my CV. If an employer asks, I'll tell him that it was in-fact a volunteer role. By leaving it off, the first impression of that role is as it should be: FT employment. Do note that my resume does not have an employment section and that this is by design.

I have also volunteered for the food-bank, my university, local homeless shelters, etc. None of these positions are particularly relevent and you won't even find them on my resume/CV. They don't contribute anything to my qualifications for a role. 

Regarding phrasing

None of the provided examples communicate anything about the nature of your work. If you decide to include these roles in what I presume is a CV, you will need to add a few words indicating what you did for them. For example if I were to tweak your provided titlees (Depending on the context, "Volunteer" is entirely optional):


Volunteer Tech Support for Institute of Electrical Electronics Engineers (IEEE)
Volunteer Webmaster for the Project Management Institute (PMI)
Volunteer Teacher for the United Nations High Commissioner for Refugees (UNHCR)


A Final Word on Honesty

It is exceptionally easy for an interviewer to know when you are "Valourizing" your experience. It would do to repeat that again. Humans are terrible at lying and your interviewer likely has at least a decade more experience than you. 

Embellishment is okay to an extent and only you will know what that is for your role. I heartily support selective truth-telling (ie changing of job titles to communicate what you really did or removing irrelevant tasks from descriptions of roles), but I am careful that I never lie on my resume or CV.