Intercept a function call angularjs factory

I need to intercept a function call from GTM through the angularjs factory.

I was able to do this for changing a variable: angular.element(document.querySelector(‘div.os-reg-form__mentor-row’)).scope().$watch(‘registrationService.userID’, function(uID){}) ;

But to call the function, watch works only once (which is logical, but the function does not change).