When setting up VOIP, the only security considerations you can manage with are those on your own network. You should make sure your network is password-protected using 128-bit encryption and sits behind a firewall. But once the VOIP traffic leaves your network, there isn't much you can do. Security moves into the hands of the network provider and the person receiving your call on the other end. If you're connecting VOIP to your data network, your IT team or technical consultant should help you deal with setting up proper security precautions.