Why doesn't ObiTalk work with Google Fi?

Episode 1559 (34:50)

Johnny from Atlanta, GA
Rotary phone

Johnny uses Google Voice and bought an ObiTalk to use with some older phones. Is that a good idea? Leo says he used the ObiTalk for a long time. It's very cool. However, It doesn't work with Google Fi, so if his Google Voice is tied to Google Fi, he'll have issues. Johnny would have to use a second Google Voice number, but that doesn't do him much good and he can't forward that number to his other Fi account.