Ask HN: How to craft an effective technical phone screen?

I'm going to work on building out a technical phone screen (ie. Interview before you're invited to a virtual on-site) soon targeted at full stack and backend product software engineers(mid-senior level) and wanted to see what topics/concepts have worked for folks in the past.

I'm also interested in experiences/questions that interviewees had that were enjoyable.

My initial thoughts are that I'm avoiding leetcode type questions and I'm going to try to craft a question based on a hard technical problem I experienced recently.

For example, loading up a bunch of data into memory and optimizing a slow function, then as a second part I would create some space constraints which would require some changes to the way the data is fetched.