Below you can find step-by-step guide on how to add area codes to your contact numbers according to the country they are registered in:
Open the Contacts Optimizer app and select your contact account
In main menu click on "Toolbox" and then on "Internationalization"
Select the desired country or click on "Advanced" and specify your code manually. You can also use the Advanced feature in order to change larger portions of the number (other than just the one in the country prefix)
Select the contact from your list you want to apply the area codes to and click on "Next". Then confirm the action by clicking on "Save changes" and let the operation finish