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

{
  "age": 242352,
  "content": {
    "version" : 0,
    "call_id": "12345",
    "lifetime": 60000,
    "offer": {
      "type" : "offer",
      "sdp" : "v=0\r\no=- 6584580628695956864 2 IN IP4 127.0.0.1[...]"
    }
  },
  "origin_server_ts": 1431961217939,
  "event_id": "$WLGTSEFSEF:localhost",
  "type": "m.call.invite",
  "room_id": "!Cuyf34gef24t:localhost",
  "sender": "@example:localhost"
}