texts to Isle of Man charged as international?

Hello,
I'm trying to figure out if the UK networks charge international
texting rates for texts to the Isle of Man. H3G definitely does. Virgin
Mobile definitely does not. However with the others, T-mobile, Orange,
Vodafone, O2, they say they do on their websites, but I heard that
people who texted IOM numbers were only charged for a domestic text.

Furthermore, since Virgin operates on T-mobile's network, how is it
that Virgin can charge IOM as domestic, while T-mobile charges it as
international...it's the same network!!

I would appreciate feedback from anyone who has texted the Isle of Man
and knows what they were charged.

thank you,
Mario