If you have an existing Google Voice number that you'd like to use with LibraryH3lp, it's very simple to forward those texts to your LibraryH3lp SMS add-on number. Your users will only see your GV number. The forwarding is nearly instant and works very well.
Historically speaking, our SMS technologies have evolved over the years. We started with a custom Android app because, well, that was the only way to get things done. Later, after GrandCentral became publicly available as Google Voice, we were able to use it for SMS. The use of Google Voice for SMS was never officially sanctioned by Google and it has become well-nigh impossible to get it to work with new accounts. (Old accounts still function.)