Apart from the usual checks that Roaming is turned on and International Call baring off. The only other thing to check is the SIM Toolkit. Set Roaming to International rather than automatic
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Thank you for reaching out to us in our community and welcome, sorry to hear Roaming isn't working, did you make sure this was active on your online account before you left the UK?
Had the same problem for 5 days. Then realised I hadn’t switched the Roaming on on my Virgin account app. I had already switched it on in phone settings.