I suspect that as Virgin probably has limited coverage abroad from their own network they would have to rent airtime from the established networks, which obviously they are reluctant to do. This is the only reasonable explanation in my humble opinion, unless of course you know differently?
This problem has nothing to do with international barring, thats for calling out from the UK but it does not matter if its on or off. Two phones, both sim only contracts, in the same room. One is connecting, the other is not. Try to connect manually and it says 'limited connectivity'. Checked account and both numbers have roaming on. Ring 789 was the advice from the Virgin shop and also from the sticky post at the top of this forum but we did not have to ring anyone about the phone that is working. Swapped sims between phones to eliminate the possibility of a phone setting and as expected, the working sim continued to work. The only difference between the two is that the one not working ported over its old O2 number.