On calls I will try to use their first name, unless there are multiple people with the same first name there as well. Then I just refer to people by the NATO phonetic alphabet of their last name. I do ask for their permission before I do so though, I'm not trying to be a jerk :)
In a DM, if I tell you @fred can help you if you have any questions, you can now hover to see Fred's availability status, and click to message them.
An added benefit is that these tags track the user's displayed name. If Fred changes their name to Francine, the places where you've tagged Francine in the past will update automatically. You can also search for places where users are mentioned this same way.
Same thing with email rules prioritizing mail that has the recipient on the To: line.