vodafone had me pay about 50 usd a couple of years ago (back when i had my old n95 8gb) forr some alledged extra internet trafic.. even on the bill there were a lot of around 170 mb downloads that i couldnt possibly have made so i confronted them with the phone's log that showed under 500 mb total for that month so eventually after 3 months without phone they admitted their mistake but i had to buy a prepaid sim meanwhile because my subscription was on hold...phew
anyway long story short its possible that they made a billin mistake just dont forget NOT TO ERASE THE LOG oherwise ...
good luck!