Usual not giving all the facts when selling.
The O2 SIM will be chargeable. They really should have told you how much the total charge would be, split between a payment to Virgin Media and another to O2.
Only O2 can tell you their cost and what package you are on in terms of data.
If you use the Oomph SIM you can get a PAC code to swop your number to O2.
https://community.o2.co.uk/t5/Volt/Volt-Megaguide/td-p/1541963
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