Problem call number in app

ionic-v1
1

Hi all
I have a problem, the app does not link the number of my factory. I have the code:

<a href="#" class="button button-balanced" onclick="window.open('tel:{{item.num}}', '_system', 'location=yes');">  
        <i class="ion-ios-telephone"></i>
</a>

If I put a number instead of {{item.num}} , it works. Any ideas? Will be syntax error? I use Cordova with Ionic, and the plugin InAppBrowser.

Thanks, as always.