Well, like I said, it's a sporadic problem that has only happened 3 times in the month I've been using it. And I'm not gonna be able to get her to try something different (it's her cell phone). However, it has never happened to me before, ever, before I switch to Obi & GV, so I assumed it had to be related.
What component in the chain is responsible for generating these tones: her phone/device (a cell phone), her carrier (verizon, I think), my carrier (Google Voice) or my phone/device (Obi). It sounds like you are saying it's one of the first two. If so, that's a little odd that she's had both the same phone and same service for years without ever encountering this, but if that's what it is, then that's what it is.
Is there any possibility that it was my end that was generating it, perhaps in response to my wife's voice (I don't recall if it was my wife or her mom that was speaking at the moment it occurred). Would I have heard the tone on my end, too? If it happens again, I'll try to pay more attention to exactly what's going on at the time.