set IPHONE to do shell script "if /usr/local/bin/arp-scan -l | grep your-iphone-mac; then echo 1; else echo 0; fi" user name "your-username" password "your-password" with administrator privileges
tell application "System Events"
    tell process "Messages"
        tell menu bar 1
            tell menu bar item "Messages"
                tell menu "Messages"
                    tell menu item "My Status"
                        tell menu "My Status"
                            if IPHONE is not equal to "0" then
                                click menu item "Available"
                            else
                                click menu item "Away"
                            end if
                        end tell
                    end tell
                end tell
             end tell
        end tell
    end tell
end tell

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
  <key>Label</key>
  <string>com.your-username.checkiphone</string>

  <key>ProgramArguments</key>
  <array>
    <string>/usr/bin/osascript</string>
    <string>/Users/your-username/bin/check-iphone-available.scpt</string>
  </array>

  <key>Nice</key>
  <integer>1</integer>

  <key>StartInterval</key>
  <integer>60</integer>

  <key>RunAtLoad</key>
  <true/>

  <key>StandardErrorPath</key>
  <string>/tmp/com.your-username.checkiphone-available.err</string>

  <key>StandardOutPath</key>
  <string>/tmp/com.your-username.checkiphone-available.out</string>
</dict>
</plist>

launchctl load com.your-username.checkiphone.plist