Call Functionality Issue

It is a kind of skype application. When the app goes background, after 15/20 mins when trying to call anyone of the user's extension using call functionality, I can hear the message "user is not available". I have used SIP.js and free switch for this call functionality and also i am using background timer for android and VOIP push notification for IOS. I want my app to work eventhough it was killed and also till logout.

registerGlobals();
      const userAgent = new UserAgent(
        this.getUserAgentOptions(extension, password)
      );

      console.log("!!! SIP initiaing useragent.start");
      userAgent.start().then(() => {
        console.log("!!! SIP inside useragent.start");
        const registerer = new Registerer(userAgent, {
          expires: 120,
          refreshFrequency: 95,
          logConfiguration: true,
        });
        registerer.register(); 

        // backgroundsync
        BackgroundTimer.runBackgroundTimer(() => {
          console.log("$$$ background timer before$$$");

          this.isBackgroundSyncStarted = true;
          // this.loadData(true);
          registerer.register();
          // code that will be called every 3 seconds
          console.log("$$$ background timer after$$$");
        }, 20 * 1000);
        if(Platform.OS == "ios"){
          BackgroundTimer.start();
        }
        console.log("!!! SIP registerer registered");
      });