Thanks to everyone for their responses. I've gotten more information than I could have hoped for!!
This is being done on a client's computer, not my own, and I don't have access to it at the moment. But my memory is half decent on good days, so . . .
We are using the Google Voice plug-in, not Hangouts. On my own computer I have the Voice plug-in under Firefox for one GV number and the Hangouts App/extension under Chrome for another. On the client's machine I believe it's Voice and Firefox.
I have already reported this issue as best I can to both Freedom Scientific and Google. I am hoping that Google will take the accessibility issue seriously and look into this.
In the meantime I think the work around we'll end up using is having her Google Voice number ring her cell number on her iPhone. I'll probably also try to assign some sort of dedicated ringtone to those calls, but I've got to figure out the best way to do that, so that she knows whether the call is coming in directly to her actual cell number or being forwarded from GV.
It will probably not be until after the holidays that we next get together and that I can do this, but her internship won't start until that time, either. I try to avoid working "at deadline" for precisely these reasons. Having done this sort of work for some years now I have long ago been disabused of the notion that JAWS, or any other assistive software, is likely going to behave in the way I want it to, particularly with new or obscure features. Given the pace of change in web browsers and general web coding it's a miracle that the assistive technology companies can keep up with it at all. I just wonder how long it will be until stuff becomes stable with Windows 10!