One of the great benefits of using the phone abroad is you're not limited to a single network provider.
More often than not you can leave the phone set to 'automatic selection' as the network offering the best signal will then be selected.
Of course if you do have issues, you can manually select from the available networks listed.
In rare cases, some networks may not allow you to connect manually if there is no roaming agreement in place or if that network is experiencing a fault in that are. In this case, you may need to choose an alternative.
Just click here for more information about roaming and the costs and coverage of where you're visiting.