But the isle of Man is neither part of the UK nor Europe.
It is part of the UK for telecom purposes.
The IOM dialing code is a UK code
By this same reasoning, Canada is part of the United States. But that is clearly not the case. Neither should the Isle of Man be automatically assumed to be part of the UK or the EU.
Just to confirm, it is - I've just spoken to my parents on the Isle of Man using the Unlimited UK & Ireland Subscription.