Yes. I am very sure dialing 789 is free from anywhere. That's the same number you dial to check your balance. The automated answer picks up and tells you your balance, then it tells you your options for topping up. Voucher, CC, etc. You choose 1 and it asks for the 16 digit voucher#, and credits it right away. I have topped up several times mostly from outside the US.
From their website, under call costs while roaming, it clearly states 789 is free.
http://www.virginmobile.com/mobile/service...ff_coverage.jsp
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Sim cards: AT&T (Contract), 3 UK, Piranha Mobile