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 mobile provider for details about international roaming countries, SIMs, rates, and policies. If you do not have an international data plan, turn off Data Services While Roaming to avoid data roaming charges.
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.
If data roaming is enabled on your device, we suggest that you set Media auto-download to WiFi only to avoid extra data charges. To know how to configure media auto-download please read here.
We 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.