If your phone is locked, you'll see an incoming WhatsApp Audio... screen when someone calls you, where you can:
- Slide to answer the call.
- Tap Remind Me, then select if you want to be reminded When I leave or In 1 hour.
- Tap Message to decline the call with a note.
- Decline the call by pressing the power button on the side of the iPhone two times.
https://faq.whatsapp.com/1153602608602452/?cms_platform=iphone&helpref=platform_switcher
Note that is the page for iPhone and WhatsApp also says that all features in the iPhone version are not available on the iPad version of WhatsApp.
- Linked devices may not have all the features found on WhatsApp for your phone.
https://faq.whatsapp.com/329727446319089
For additional help with WhatsApp, you can look here, although their customer service is not very good and they do not have an option to call for support and they do not offer a support forum such as this for their users to discuss issues.
https://faq.whatsapp.com/