Open WhatsApp
Go to the chat of the person you want to call
Tap the call icon
Turn on Silent mode or Do Not Disturb on your phone before the call
Lower your phone volume to zero
Use headphones if you want to reduce sound further
If you mean muting incoming WhatsApp call notifications, go to WhatsApp settings and disable call notifications if available on your device
If you mean silencing a specific chat, open the chat, tap the contact name, and mute notifications