Most enterprise organisations encrypt data that transverses the network. It's a no-brainer. Thus, encrypting voice and data packets transmitted over a VoIP network is also a given, right? Turns out, not so much. By and large, VoIP vendors are building encryption support into their offerings.
http://www.computerweekly.com/Articles/2006/10/02/218505/Fast-Guide-VoIP-encryption.htm
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http://searchnetworkingchannel.bitpipe.com/rlist/term/LAN-Telephony.html