Probably your SIM, though getting Virgin to admit it will be a challenge
We recently travelled with TWO phones - both Virgin. My wife’s roams fine, mine doesn’t. (But it did work with manual roaming enabled)
However, when I switched SIMs, my phone started working just fine and my wife’s STOPPED working
If that doesn’t demonstrate a SIM issue, I don’t know what does!!
They’ve FINALLY sent me a new SIM, but I won’t be back to the island for a few months, so no easy way to test it