Both numbers needs to be provisioned for this service (most numbers should be already)
Both users needs to be on the 3G network
Both users needs a video calling capable handset (just because a phone has a front facing camera ... does not automaticaly mean it is capable of a video call I think).
We have two people here on S2`s. Both in 3g coverage. Both phones have the function for a video call. Its not working.