How to create and call custom functions on symbols?

It is pretty clear how to assign mouse and timeline events on symbols, but I can't find any examples of creating a custom function on a symbol, and then calling it from the stage or another symbol. I can make this work wth time-based triggers, but I'd like to just call a function like... sym.getSymbol('manager').doSomethingCustom();

Putting stuff in the onCreationComplete for the symbol like:

function doSomethingCustom(){

     console.log('Custom stuff happening');

}

And then calling in from the stage like:

sym.getSymbol('manager').doSomethingCustom();

... just nets a "Timeline error" (these are a b*tch to debug, BTW.)

So how is this done in Edge compatible syntax?