Did you try updating the Phone app on the Galaxy Store? Also, check under Battery settings to see if the Phone app is 'allowed' to be used without battery restrictions. Otherwise, this looks like a bug in the Phone app or system software. Try checking for Software Update, the issue may resolve after the update. Or if the issue arose after a recent update to the Phone app, or system software update, only the next update will fix this. Finally, if the issue persists, try using another phone app from Play Store such as Google Phone.
Go to Settings > Security & Privacy > more settings > Permissions Manager. Then look for Call. Isme dekhein ke WhatsApp 'Not Allowed' list men hai to usko 'Allow' karein.
