Hi M_Apple98,
Thanks for reaching out in Apple Support Communities. I understand you're hearing static when placing a call on your iPhone that doesn't occur when you use WhatsApp or other calling apps. I also see that this sound persists when testing with headphones as well. I can see why you'd reach out about this behavior; I'd like to help. I'd recommend using the following article to troubleshoot this behavior; although the article title doesn't exactly match your situation, I believe the steps included may be useful: If you can't hear a person on a call or voicemail or if the sound isn't clear on your iPhone
I'd also suggest reaching out to your carrier, if you haven't already, to ensure all is well on their end.
If you continue to experience this issue after using the steps provided and reaching out to your carrier, I'd suggest contacting Apple for additional assistance: Get Support
Best Regards.