Google Voice is not a way to reduce prison phone call costs. The prison will block the calls. Don't even waste your time with this.
There is apparently "prison lore" about using Google Voice for inmate calls, in that we've seen this question for years, thousands of times.
At one point a few years ago, the FCC was considering forcing the contractors who supply inmate calling service to not charge exorbitant calling rates (i.e. charge no more than the prevailing rate for the public to make a call, plus some fixed fee for the prison infrastructure). One of the largest inmate calling companies lobbied so hard that the FCC apparently gave up. Blame the evil parasites who are ripping off vulnerable families of inmates.