If you want to be truely secure, passing all of your phone conversations via internet is probably not the safest route.

Why Nobody’s VoIP Is Secure
http://www.voip-news.com/feature/voip-insecurity-111907/

VoIP maybe not so secure?
http://arstechnica.com/news.ars/post/20060407-6552.html

In a recent Wired article, Bruce Schneier highlights a threat to VoIP that I rarely see considered in news coverage of technology: call endpoints are vulnerable to local, PC-level compromise, and VoIP network traffic is vulnerable to everyone from the feds to criminals.

How Secure Are Your VoIP Calls?
http://www.voip-news.com/feature/how-secure-voip-calls-010207/
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Security is relative, your only hope is to super encrypt (encrypt something before you send it through an encrypted channel) the parts of your voice network that are in the public network.

For example, my office has a stand alone VoIP server. It is designed so it can't talk outside of the network through the VoIP lines. In order to talk to the outside world, it has to go through one of our handful of phone lines.

In order to be able to use VoIP outside of our network, you will need a VPN connection.

Problems? You can still tap one of the physical landlines. If the place that hosts our VoIP server has weak security, you could bypass the VPN since you could connect local to the network. If the VPN software used to connect the phones remotely is compromised, that also gains you access.

On top of that, there could undisclosed vulnerabilities in the VoIP server itself.

Basically, nothing is truly secure. It may take a day, a week, or years, but everything can be broken given enough time and resources.
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