If you have received the bill all outgoing calls will be itemised. Outgoing calls are logged against the IMEI number of the phone, and there would be no doubt by Vodafone the calls originated from the phone and SIM.
You need to ask yourself f there is any chance someone has had the opportunity to use your phone, it's easy to delete an outgoing call from the phone call log. You need to add all the security measures possible the phone offers to prevent unauthorised use. From your online account there are bars you can add, this includes an international calling bar restriction.
It might be worth Googling the numbers to see if there are any other instances reported elsewhere - scams can be so sophisticated they cause you to lose money without realising what is happening.
The best thing to do is to let the Social Team get this investigated on your behalf, they can be contacted through social channels by following this link Contact Us .
