Found this thread at Reddit which has links to two tweaks that, once installed, will allow the caller to choose Google Voice as the calling method. What it actually does for the "Google Voice" option is route the call via Hangouts, which must be installed. So...the call won't be going out via your phone line as Hangouts calls go out over wifi or cell data. I installed them via iFile. I was a user of the "Call on GV" tweak that was never updated past iOS 8.