Outgoing SIP provider with no registration? More info

[LG1]

Hi All,

Looking at Vaxalot and it sounds great. Just looking at the basic features and im unsure of this:

"Note: Some Voice Service Providers require a SIP registration before an outbound call can be made. A VoxPremium account is required for members that wish to make outbound calls using these types of providers."

Just trying to find more info about SIP providers with no registration. Some things I would like to know:
What is an example of such a provider?
What authentication/security measures does this method provide?
Does anyone use this account(voxbasic) to access PSTN? and how?

I have been searching and haven't turned up much, hopefully someone on here with experience can save me some time.

Cheers