core/homeassistant/components/fritzbox_callmonitor/icons.json

{
  "entity": {
    "sensor": {
      "fritzbox_callmonitor": {
        "default": "mdi:phone",
        "state": {
          "ringing": "mdi:phone-incoming",
          "dialing": "mdi:phone-outgoing",
          "talking": "mdi:phone-in-talk"
        }
      }
    }
  }
}