Hi Yvos,
Thanks for coming back to us on this one.
The 3 digit number for calling Virgin Mobile is 789. This is free of charge and doesn't cost anything however you didn't dial this number on the occasion you were charged.
The number we advised you on via PM was dialled from your handset with the SIM connected. There is no other way the number could have been charged.
I know from experience that this can sometimes happen if you are unsure on a number to call a company and use a search engine to try and find it which is why I advised to always check our contact us page.
If you have any further issues, pop back and let us know.
Many thanks,