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.
To confirm that your data roaming is disabled:
Go to Settings > Wireless & networks > Mobile networks and make sure Data roaming is disabled.
Go to Settings > More (under Wireless & networks) > Mobile or Cellular networks > and make sure Data roaming is disabled.
Look for Network or Data in your phone settings and disable the Data roaming option.
Provided you have Wi-Fi internet access and active WhatsApp service, WhatsApp should be free to use over a Wi-Fi connection.
If data roaming is enabled on your device, we suggest that you disable all media in When roaming under Media auto-download in your WhatsApp settings to avoid extra data charges. To learn how to configure Media auto-download please read here.
While traveling, 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.
WhatsApp Support Team