The same Consumer Protection restrictions should apply to ringless voicemail as to other calls.

It’s still a call, whether the phone rings or whether the call goes directly to voicemail. To claim otherwise (as "a provider of the service" is attempting to do) is self-serving bullshit of the purest order (ordure!) and should be discarded as such.

If the current laws don't address this variation (because it did not exist at the time the laws were written), then new legislation (or interpretation of legislation) should clarify: ringless voicemail falls under the restrictions, and any new forms (e.g. voice-to-text, text-to-voice, text-to-sign-language, videos) are equally (and pre-emptively) included.

People who sign on to the Do Not Call list DO NOT WANT TO BE BOTHERED, no thank you very much, so just bugg off!