When a message is sent to you via WhatsApp, but you do not have the app open to receive it, your iPhone will automatically display a push notification to alert you of the message. iOS apps can provide three types of notifications:
There are two ways to answer a WhatsApp call based on your notification settings:
Please verify that your notification settings are turned on in both your WhatsApp and iPhone settings:
The alert sound volume is controlled by your iPhone's ringer volume, which can be set in iPhone Settings > Sounds. Here, you can also set your Vibrate preferences.
If you have verified that your notification settings are correct in both iPhone settings and WhatsApp settings and you do not receive notifications, this is more than likely an issue with your connection, iOS or Apple's Push Notification Service.
Please note that delivery of notifications is controlled entirely by Apple's Push Notification Service (APNS), and WhatsApp has no way to resolve issues with this service. If someone sends you a message while you are offline, that message will be sent to APNS to be delivered to your phone. WhatsApp has no control or visibility over the delivery of these notifications. The issue may be presenting itself in WhatsApp, but the issue derives from APNS or the iOS.
Typically, the only way to resolve this issue is to restore the phone to factory settings and set the phone up as new. If you restore a backup, you will likely restore the issue as well.
Also note that push notifications require a valid SIM card and an active Wi-Fi or cellular connection.
WhatsApp Support Team