title: Firebase merge similar notifications in Android
body: <p>We are using Firebase in a SIP app to send us missed call notifications and chat notifications whenever the app was offline.</p>
  
  <p>While sending and receiving works fine, we have the effect on the Android client, that 5 missed calls obv produce 5 missed-call-notifications, filling up the notification bar on the client device.</p>
  
  <p>How can we merge those notifications together, to just show a single "5 missed calls" notification?</p>
  
  <p>Is there any additional flag (like grouping) we can put in the <code>data</code> or <code>notification</code> part of the message?</p>
  
  <p>Here is an example of our current missed call notification:</p>
  
  <pre><code>{
      "to":"&lt;&lt;FCMToken&gt;&gt;",
      "priority":"high",
      "notification":{
          "title":"&lt;&lt;Displayname-of-Caller&gt;&gt;",
          "text":"&lt;&lt;Date-and-time-of-call&gt;&gt;", 
          "icon":"icon_notification_missed",
          "click_action":"MISSED_CALL"
      },
      "data":{
          "type":"sip-call-missed"
      }
  }
  </code></pre>
  
  <p>So what's the trick in combining them together as one?</p>