Q: how can i cancel a call back

You should have received an email from Apple about the call - clicking on the Edit button should take you to where you can sign in and cancel the call

We are all end users here

Well - that is unusual to say the least usually when you schedule a call or chat - they let you know about it

You could call Applecare, provide your case number, and they should be able to see a scheduled callback, and cancel it for you.

AppleCare US phone number: (800)-694-7466

https://support.apple.com/en-us/HT201232

Chances are, if you did not get the email, your scheduled call somehow did not get scheduled

Calling AppleCare like ssschmidt suggested would be a way to confirm or deny or you could just leave well alone

Got the email, signed in, got Case and Repair Details window, but there's no place to cancel the call.