There are some good tips here, but two stand out as terrible advice:

1. "Chances are you’ll be blindsided and thus less able to advance your own interest."

This isn't about you; it's about your boss, who needs your help. If they had wanted to write it all down, they would have emailed rather than call. By all means send a quick text/PM to ask for the topic before calling to be better prepared, but if you're worried about being ambushed, you might want to look for another boss.

3. No, I am not going to set myself a reminder to send an asynchronous communication at your convenience. I emailed because emails are, by definition, at mutual convenience. If I know you're not going to read it, I'll send the message instead to someone else (probably your boss) and privately refer to you as "that self-important jackass" for the next three years.