matrix-doc/event-schemas/examples/m.call.invite

{
  "$ref": "core/room_event.json",
  "type": "m.call.invite",
  "content": {
    "version" : 0,
    "call_id": "12345",
    "lifetime": 60000,
    "offer": {
      "type" : "offer",
      "sdp" : "v=0\r\no=- 6584580628695956864 2 IN IP4 127.0.0.1[...]"
    }
  }
}