Cingular phone unlocked, can't make calls

Hi,
I just popped a prepaid Vodafone sim from Europe in my 3100. The good news: The phone was not even locked. The sim card was accepted. The bad news: I can't make any calls.With ever number I try, I get this "The call cannot be made from this line" voice. However, when I type *100# I get the proper balancece on my prepaid account. Does any know what the problem is???

Thanks,
revengineer