The number I would like to call is TNT - 199803868
http://www.tnt.com/express/it_it/site/home/Numero-unico.html…
Tried calling +39 199803868 with no luck ("The number you have called is not in service. Please check the number you have dialled"). I'm assuming it would be like trying to call a 1300 number from overseas where a simple +61 1300 xxx will not work
+39 from a mobile will work just as well.
The + does that
Think you'll find calling a 1300 number from outside Italy is not achievable.
Try to find an alternate number.
Normally they'd provide an alternative to the so-called unique number for international callers. TNT Italy is just being difficult.
You'll have to use the contact form as they suggested.
You can try Skype which often caters to special, premium, and non-geographic numbers, but for Italy there was no additional information given so maybe it won't work. Unfortunately the free month trial of Skype doesn't cover those special numbers anyway (assuming Skype treats them as such or allows them at all).
Make sure you also include the exit code (0011) at the beginning of the number.