First, check with your service provider if you have access overseas and account is active. i.e roam is on and you have reception.
If everything is in order and you can receive calls but you can't make outgoing calls where you believe you can (i.e if you use the same SIM in another phone and you can make phone calls) then most likely is the Call management setting is incorrect.
In your device go to Option > Call Management > Smart Dialing, and you will see a pre-setted country code from your country. This means every call you make this country code will be automatically added to the number you are dialing.
Change the country code to where you are, this will enable to make local calls to the country you are in.
However, if you want to make an international call back home you will need to type in the country code in front of the number just like you will need to when making a international call from a payphone or any other mobile.
Hope this helps!!!!