CallKit lets you receive and answer Snapchat calls without first unlocking your device or opening the app. CallKit is a feature that’s built into Snapchat. Calls made through CallKit don’t show up on your phone bill.
Snapchatters on iOS can also call friends back from their device's call log.
To prevent calls from non-Friends, you will need to update your Privacy Settings options for 'Who Can Contact Me' and 'Send Me Notifications.'
Please Note: CallKit can’t be disabled, but you can delete entries from your iOS call log.