When I travel to another country, will I be charged for data roaming?
WhatsApp Messenger uses the same internet data plan as web browsing and email on your phone. If using your cellular data while roaming will normally result in additional charges, you may be charged extra by your mobile provider for using WhatsApp while roaming. Contact your wireless carrier for details about international-roaming countries, SIMs, rates, and policies.
If you do not have an international data plan, turn off Data Roaming to avoid data roaming charges in Windows Phone settings > system > cellular > Data roaming options > don't roam.
Using WhatsApp while your phone is connected to Wi-Fi should not incur charges from your carrier. Please note that some Wi-Fi networks may have charges or usage fees.
We also recommend that you set your WhatsApp status to "I am currently roaming abroad, will reply when I am on Wi-Fi" -- this way your friends will know why you are not answering right away.
WhatsApp Support Team