Around Christmas time, go buy a phone from Costco…phone only. If yours stops working, you can then use the Costco phone. If it doesn’t, you can return things to Costco for longer than most other places.
I just replaced my old Flip Fone, mainly as i got a new smart phone for free.
I think you need a new cell phone company! 
It looks like her Honor 6X does not support VoLTE.
I am OK getting her a new phone, but is it really true I can’t get a Huwei or Xiomi with the Google Play store on it? That’s really disappointing.
I’m not sure what to get her.
Lots of good options depending what you want to spend. I personally use the Pixel 3a (not sold anymore, but the 4a is) which is under $400 and has a good camera and performance. But if you’ve bought phones from ebay before, sounds like you want a very basic android based smartphone.
Maybe something like this?
It’s around $100 - should get you most of what you want. I do advise against buying phones on ebay unless the party has a HUGE amount of business and positive reviews, I have endless horror stories of people buying phones that way and finding out that they’re locked as the person who was financing them just stopped paying after they ‘sold’ it, which IMEI blocks it, or it was reported ‘stolen’ or ‘lost’ with the same result.