Video Calling -- what UK networks are compatible with each other?

Does anyone know what networks work with each other for video calling?

From memory, I once was able to use my T-Mobile MDA Pro to make a video call to my friend Jo’s Vodafone (and vice versa). So that would indicate Vodafone — T-Mobile connectivity.

Can Orange users call T-Mobile users? Or Three users call Vodafone?

Anyone know?