spurious calls made and added onto my mobile bill.

[Dm999]

 I recently purchased a sim only package, tranferred the number from EE to Sky exactly as instructed. Activated the sim into the new phone I had purchased, everything seemed fine. Sky took payment for the first bill as normal ( which was just £6 for the cheapest sim only deal ),  I logged in to my sky account to look at the bill and saw on the next bill that there were charges of approximately £87 for calls abroad that we had not made. Please, can anyone tell me if they have had a similar experience.