The default background when someone calls you is your lock screen photo. To prevent the default from showing up, you must get a call from a contact who you set a picture for. This means setting a picture for say your mother's contact would mean the picture you set for her will show up in the background instead of the lock screen picture. You can set pictures to contacts by going into the Contacts or Phone app and tapping a contact and pressing the edit button in the top right corner to add a picture from your phone's photos. So if you really hate your lock screen photo showing up when you call, set contact photos for everyone in your address book (there is no quick way of doing this). You will then never have to see your lockscreen photo in calls unless you get a call from a non-contact, private, or blocked phone number. Hope this helps, and I will definitely try sending a request to Apple to create a default phone wallpaper.
Thank you for choosing Apple.😉