Examine source code of Velantro Click to Call

{
  "manifest_version": 2,
  "name": "Velantro Click to Call",
  "description": "Velantro.net PBX click to call extension",
  "version": "1.4.12resigned1",
  "applications": {
    "gecko": {
      "id": "{2a7e3959-cfd5-4f2b-a537-85bdbe4f6cbe}",
      "strict_min_version": "57.0"
    }
  },
  "browser_action": {
    "default_icon": "velantro32.png",
    "default_popup": "popup.html"
  },
  "web_accessible_resources": [
    "call-phone-v.png"
  ],
  "options_page": "options.html",
  "options_ui": {
    "page": "options.html",
    "chrome_style": false
  },
  "content_scripts": [
    {
      "matches": [
        "http://*/*",
        "https://*/*"
      ],
      "js": [
        "phonepattern.js",
        "render.js",
        "monitor.js",
        "render-init.js"
      ]
    }
  ],
  "background": {
    "scripts": [
      "velantro-storage.js",
      "velantro-config.js",
      "velantro-call.js",
      "velantro-events.js"
    ]
  },
  "permissions": [
    "activeTab",
    "storage",
    "notifications",
    "contextMenus",
    "http://*/*",
    "https://*/*"
  ],
  "icons": {
    "16": "velantro16.png",
    "48": "velantro48.png",
    "128": "velantro128.png"
  }
}