The auto dial of international prefix(0039) not working

Enable the international city/country prefix rules in your Fritz!Box and use proper city and country code in your settings for the sipdiscount account you defined. It will work

The combination should be country code = 0039, country code prefix = 00, keep country code prefix = NO.

Don't try to mess with the settings on "Wahlregeln"/"Dialling Rules". If you enter any fixed provider prefix there, this may screw up the numbers actually dialled.

Cheers,
--gandalf.

German voice of menu? Hmm...

Step 1: install the firmware patch to open international features. You get the patch from https://www.the-construct.com/traymessage/index.php?page=costum&lang=en

Step 2:
In the Internet Telephony configuration where you define your providers, you'll find an additional option to specify country codes and prefixes: "Landeskennzahl", "Landeskennzahl-Prefix".