Most of the jail and prison systems I know have technical checks in place for Google voice and other VOIP systems to avoid you getting around their charges. They will ban the numbers and then often ban you from the phone system and sometimes put you in the Hole for games like that.

I used to use them to try to call the UK instead of paying multiple dollars a minute when my mother was dying of cancer. But it's a game of cat-and-mouse. And if you're in a place that makes you wait 4-8 weeks to get a number added to your call list, then you can't afford your number to get banned.

Every day I help multiple guys inside do "3-way" calls from prison to numbers that aren't approved onto their lists yet. It's a dangerous game, though, as the calls are often detected and blocked.