Try to make an investigation on your own first. Try to check the number and time the calls were made, its all in the statement isn't it? Maybe there is really someone using the phone when youre outside the country. **creepy
ask your relatives as well, maybe they used the phone and forgot to notify you about it.
I investigated. The numbers that were called, were either Smart or Sun numbers, which me or any of my family members do not have. It would be impossible for someone to make a call while we are out, as no one is left at home. The only possible problem I see, is either a cross line, as last time I was using the phone, someone was online dialling a number. I also pointed this concern to the customer service. Anyway, thank you for your help.