buildResponderView.dart

 Widget _buildResponderView(Responder responder) {
      return Padding(
        padding: const EdgeInsets.all(8.0),
        child: Column(
          mainAxisAlignment: MainAxisAlignment.start,
            children: [
          Text(
            responder.name,
            style: TextStyle(
              color: responder.isBusy() ? Colors.red : Colors.green,
              fontWeight: FontWeight.bold
            ),
          ),
          IconButton(
            icon: Icon(Icons.call_end),
            onPressed: responder.isBusy() ? responder.endCurrentCall : null,
          )
        ]),
      );
    }