> Using a cellphone is voluntarily, too.

Indeed. But the difference is that it is legal (and possible) to program a very open (and privacy-conscious) messaging service.

On the other hand, it is completely illegal (in the US by FCC ruling) to develop a completely free and open baseband processor and use it. It is also nearly impossible to set up an alternative cellphone network since you need licenses for using the parts of the spectrum.

This is where the difference is.

You can develop a phone using open band (CB radio for example).

At least in Germany, where I live, there are similar regulations concerning permission to build your own radio device for CB band as for baseband processors (i.e. if you want to legally use your self-built device there are strong restrictions (CE certifications are nearly unaffordable for makers).

Also in CB bands you are not allowed to encrypt the sent data. Thus not an option for people caring about privacy.

In the US, for pretty much all open frequencies that require either no license, a de facto license (owning a device gives you a license, CB), or a certification test based license do not allow any sort of scrambling or encryption unless the gov't is able to decode what you are sending. A third party successfully listening in defeats the entire purpose of obscuring what you are saying.

But, if you can afford an expensive license from the FCC for operating on other (commercial) bands, you can secure your communications from prying eyes.

Post office really isn't voluntary. When I get my required tax forms from France, if I don't have the post office, I'm unable to complete an action required by the State. That means the Post Office isn't voluntary unless you want to break the law.

When my kid's school requires What's App in order to submit required documents, then there might be a claim that What's App rises to the importance of the Post Office or the telephone.

Right. However there's many competing post services providers in my country, and the ones used by the government and institutions change from time to time (there's open competition every few years and for a while courts were using different provider than the usual national post for example). And no matter which one was "mandatory" at any given time - all of them have to keep the secret of correspondence and have to comply with the law regarding storing private informations etc.

ING, a bank, is starting to require WhatsApp instead of email for customer service, in the Netherlands I think.

You're not that far off ;-)

Such a requirement would terminate my relationship with them immediately...

What if every bank started requiring it?