I had only ever dealt with Apple phone support once, back around the iPhone 6 plus orders, and I had the same thing. Got an email that there was an issue with preorder almost immediately, called in, they checked some stuff, said they'd call back, never did. Tried again, same thing. No call.
I ended up finding one at Best Buy and it was miraculously still there when I arrived through the hour of traffic.
I am unsure what their system is like but persistence seems to be the key when promised call backs with any company. TMO seems to be the worst. Every time I have an issue I am terrified if it can't be resolved then and there because I know it is unlikely to result in an actual callback. That said, credit where it's due, I called in to Tmobile asking about a trade in status saying I am ineligible and she said she would call me back when the iPhone upgrade window opened so that she could fix it manually and, amazingly, SHE DID. Two years two months I have been with them. First call back I was actually given lol.