An e-mail provider provides people with usernames for e-mail accounts. That normally means that they have access to your e-mail if it is unencrypted.
Likewise, do sip registrars have access to people's conversation when they voip each other? Or when you use your sip account to call a regular phone through a third party voip plan?
I also asked here the same thing about the voip providers, but the question was closed as a duplicate. It isn't a duplicate. My doubt is both about sip registrars -- those whose give you an address that looks like email -- and voip providers -- those whom you pay so you can not only make sip to sip calls but also sip to regular phones.
So the general question is: when you use sip for voip, who can listen and how to protect yourself?