Did you add enough credit to cover the cost of the plan that you selected? Your service won't start working unless you have added enough credit to pay for your plan, the amount you need to add is indicated on the information sheet that we sent with your SIM card.

If there are certain numbers that you can't call, then you may be trying to dial numbers that are not included in your plan, such as international mobiles or premium numbers (in case you want to vote for American Idol). If you want to make calls not included in your plan, then add extra credit to do so.

Alternatively your phone may be locked or is not compatible with the US network. You did double-check this before travelling, didn't you?