Be aware that the audio for a Voice over IP call is typically transmitted in an unencrypted format. Anyone that can capture the traffic can listen to the audio of the phone call. Luckily, Asterisk supports encrypting the media of VoIP calls. If you are using SIP, you can encrypt the media using SRTP. IAX2 supports fully encrypting calls, as well. Detailed information on encrypting media can be found in Chapter 7, Outside Connectivity.