Look, what about an Amazon Echo? I know it’s not open source at all but depending on the house size a couple Echo Dots could cover a lot. I think they can make phone calls because I’ve called my phone to find it before with mine but I’m not sure.
A benefit is that she won’t have to have it on her person.
Something like a Ring Doorbell could work too. Just thinking out loud. This is a weirder suggestion. They have a battery and nothing says they have to be installed on a wall. Just turn off motion completely. At that point she can press the button and talk to you.