I'm a little late to this topic, but since I'm here, and network security is my background, I'll throw out a few tid-bits.
SIP is an open standards protocol, very transparent and easy to intercept and play with.
This being said, all of your traffic is essentially simple to intercept, if I have access to your network.
once it's "in the cloud" tracking any given information is next to impossible, each packet has the potential to be routed slightly differently depending upon the conditions of "the internet" between you and, in this case, freephoneline.
Truth is, most people do not have physical (perhapse wireless? I hope you use WPA2 with a secure password) access to your network. However all I need to listen to your phonecalls over POTS is a handset (not a complete phone, just the handset portion) and a pair of alligator clips, then to sit down with a hard-hat, orange vest, and toolbelt next to the phonebox down the street.
There is similarly depending on location, a high probability your calls are being recorded by your phone provider, and finally, police have an over-ride that allows them to bug/listen-in on anyone's POTS calls. (with a subpoena of course... but we all know the technology has been abused.)
So generally speaking, you are AT LEAST as secure as POTS... likely more.
There are projects like SIPS (Secure SIP, kind of like HTTPS) Where the SIP packets are encrypted using an SSL certificate. Of course, SIP is not what your voice is transferred by, and is often handled in relatively slow TCP packets. SIP is only a signalling technology, routing the calls to find User A and User B, ringing phones, transferring and all the other fun stuff.
The media/voice data is transferred over RTP, which also has a secure project called SRTP (See here:
http://srtp.sourceforge.net/faq.html)
of course, security protocols and encapsulation methods add over-head which introduces it's own set of issues generally related to Quality of Service.
The easiest way to secure VoIP traffic is of course encapsulation (A secure tunnel) either from you to the far end use directly (avoiding a provider) or from you to your provider, where things will be viewed once more "in the clear" but avoids the possibility of sniffing between you and the provider.
Which brings the other issue in phone security in general.
As secure as your line might be, you are only as secure as the least secure portion of the call, be that your line, the other end of the call, or the provider in the middle. Generally speaking, I suggest you not worry about what happens to it outside your house because that's beyond your control.
When the SIP provider gets it and converts it to POTS/PSTN your security methods are instantly defeated anyway.
If you're not military you're probably over paranoid, if you need to worry about a particular conversation more than the average person, I suggest you find a way to have it in person, or write it in a complex cypher and mail/email it.