If you take off overseas and your handset does not attach to a overseas carrier you can then do a manual network selection that will force the handset to scan for networks and display available networks and enables you to select one

As stated setting your handset to manual network selection and selecting the home carrier before taking off overseas will not stop your handset from attaching to a roaming carrier once you power up your handset in another country

If anyone else says anything differently then they don't fully understand how things work