Do eval of the function
eval(func);
Try http://jsfiddle.net/99MrC/1/ for example
Update:: What your trying to do is getting the attribute value of anchor tag which is "javascript:aimsCCUAQuestionsBanner('TOPIC','VAC_FIOSTV_PROD_AVAIL','CPTU');" This will be a string and not function. So you need to eval for browser to execute the string as javascript.
When you said this is working
var func = aimsCCUAQuestionsBanner('CHANGE_CATEGORY','VAC_MAIN', '');
func();
that is because when you executed
var func = aimsCCUAQuestionsBanner('CHANGE_CATEGORY','VAC_MAIN', '');
it already executed the function.
The next call
func();
supposed to fail because it is not function, unless your function returns another function