In theory, the owner of a SIP-capable telephone handset could publish a SIP address from which they could be freely and directly reached worldwide, in much the same way that SMTP e-mail recipients may be contacted from anywhere at almost no cost to the message sender.
Anyone with a broadband connection could install a softphone (such as Ekiga) and call any of these SIP addresses for free.
In practice, various forms of network abuse are discouraging creation and publication of openly reachable SIP addresses: In the server logs, this looks like: an attempt to call a Palestinian mobile telephone (Israel, country code +972) by randomly trying 9- (a common code for an outside line from an office PBX), 011- (the overseas call prefix in the North American Numbering Plan) and 7- (on the off-chance a PBX is using it instead of 9- for an outside line).
The last hop from the proxy of the target domain to the user agent has to be secured according to local policies.
It does not provide real end-to-end security, since encryption is only hop-by-hop and every single intermediate proxy has to be trusted.