New iPhone 11 Pro Max Screeching garbled sound

Hi,

I have a new iPhone 11Pro Max, just set up yesterday. Mid call today screeching garbled sound? Don’t know what’s causing the problem? Have smart battery case on it... is that the issue. Or is it rogue apps? Read up on it earlier so turned off noise cancellation. Does technology like computers or sky tv cause an interference. Anyway it’s really upsetting with a brand new phone.

Thanks for your help.

Any guidance appreciated. Have requested Apple Service Call back but no appointment for a week.

👍