Don't forget that established users have the special hover-out user card and a bit of granular control over what it contains. Using the special summary markup, you can have your callsign display prominently in your profile page, as well as the card that expands when someone hovers over your avatar in most places. Just use:
<!-- summary: callsign here, other factoids -->
And then in your profile you can list it as you like. You're also free to make it part or all of your user name, incorporate it in your avatar somehow and probably other creative ideas I've missed.
It's fine to have a way that most people do this, communities tend to establish this kind of consistency without even realizing that they are - but we can't hope to enforce any requirements when it comes to how / if / where it's displayed.