Nokia Lumia :: 3G+ Video Calls On A 900

I bought a Lumia 900 for my girlfriend. She really wants video calling, and while, my phone is capable of it, it doesn't have the comfort of skype, so we have no choice but to use 3G+ video calling through the operator, which should be fine with my subscription, except, her Lumia doesn't seem to work with this variety of video calling. .

I can use my phone to video call other phones, but not a Windows Phone. Our operator's Customer Service agents aren't exactly familiar with Windows Phone, so they seem clueless. I can't resort to Skype-to-operator type of calling, because then I'd have to pay for Skype Credits AND her 3G. .

In a nutshell, I need ideo calling without using Skype or Tango.

Model: Nokia Lumia 900
OS Version: WP 7.5
Issue: 3G+ Video Calls (without using Skype/Tango/etc)