Now that you explained it further I agree with your idea. The learning curve is a definite with any new addition to a game and I should take that into account.
Like an idea would be a voiced "Attack (enemy champion name)!" and the ping could be a lightning bolt within a circle to symbolize attacking.
Kinda what you had in mind?
Regardless, just backing up the spirit of your OP, one of the foundations of a team game is communication. Champion counters and item builds and jungle routes are easy to analyze and memorize and pass on but being able to communicate and synergize with your team is as important and basic as knowing what your abilities do and how to use them. It's not prudent to type everything out (or even read it), so there should be an alternative and I definitely do not think VoIP is it. The vast majority of things you need to communicate with your team are simple enough to be put into a rather small script.