Given that I never get ham voice calls, it is reasonable for me to assume that any voice mails that I receive are therefore spam (but, no, you're right, I don't recall ever noticing a spam voice mail. But then I wouldn't notice one, whether it existed or not, because I don't check them)

> I never get spam voice mails

You must not be in the United States then. Pretty much guaranteed every phone number in the US has received at least one “automobile extended warranty” voicemail.