Sounds like roaming is not turned on in you account.
I've copied this across for you.
On our network, roaming is enabled automatically on Pay As You Go but if you're a Pay Monthly customer you'll find it's turned off by default. This is in place to avoid any roaming charges creeping up without you being aware of them.
If you're on our Pay Monthly service and want to enable roaming before you travel, give the team a call on 789 from a Virgin Mobile, 150 on a Virgin Media Landline or 0345 6000 789 from any other phone. You can also enable this via your online account. Please click here to log in and follow these steps:-
Online account > Plan and device > Manage services > Going abroad
If it shows it's turned off then please turn it on. You'll need to wait 2-24 hours for this to be successful.
You'd also need to turn off International call barring waiting the same 2-24 hour period. This will then enable the service to be used abroad.
If you're abroad and roaming isn't working at all for you, the same team will be able to look into it for you. They can be reached on +447953967967. The team can turn this on from their side. Once this has been done, give your phone a hard reboot (popping the battery out of the phone while it's turned on ) to allow the sim a connection refresh.
We'd also suggest a manual network search to connect to all available networks.
Apparently virgin do not recognise my details, if it isn't sorted ASAP as soon as I get back home I will be phoning and cancelling my contract. I am trying to cobby them online as I haven't got network to phone, but having trouble, will try again tomorrow as it is Sunday!!
My email and phone number is still not recognised although my phone is working (only because I am back in England), yet when i try registering it says my email is already registered, yet another mess up by virgin? I will be telephoning Virgin after the Holidays,