Phone home traffic originates from a rogue application on a device that periodically connects to a remote (usually off-network) host to receive updates or commands or deliver data collected from the infected host. The majority of phone home traffic will be the operating system and virus protection updates, Dropbox or other external services, and similar authorized and appropriate services, so it will take some effort to identify malicious traffic out of this mix.
It is important to understand the risk that phone home traffic can represent: many botnet Distributed Denial of Service (DDoS) attacks are supported by a "zombie army" of hijacked computers running software that may lie undetected for some period of time except for periodic communications with their Command and Control (C&C) servers awaiting instructions to attack a target. In a similar fashion, keylogging traffic will send periodic reports of video screenshots and keystroke data to the collecting host.
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