Tether on factory unlocked phone

I have a carrier independent iPhone 3G that I purchased in the Czech Republic.
I use it regularily on the network of Vodafone Hungary.

My data plan allows me to tether, but the tethering and MMS settings are not not active in my phone's menu. I guess this is because in Hungary T-Mobile would be the official re-seller (not Vodafone), but I am not sure...

Do you know any official way by which I can tether with my iPhone?

Thanks,
Péter