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 don’t have an international data plan, we suggest you turn off data roaming to avoid data roaming charges. If you have access to Wi-Fi and an active WhatsApp service, WhatsApp should be free to use over a Wi-Fi connection.
To turn off data roaming:
Open your phone's Settings app > Wireless & networks > Mobile networks > turn Data roaming off.
Open your phone's Settings app > Network & internet > Mobile network > turn Roaming off.
If data roaming is turned on on your device, we suggest that you turn off all media auto-download When roaming in your WhatsApp Settings to avoid extra data charges. To learn how to configure Media auto-download you can read this article.
While traveling, we also recommend that you set your About message to "I am currently roaming abroad, will reply when I am on Wi-Fi". This way your contacts will know why you’re not answering right away.