Things I’d consider are:
getting her a pet - a small dog or a cat make a fantastic companion and there are some great studies eg showing how spending time with a dog helps calm both the owner and the animal. She may find it easier to handle calling a vet, practising contact with “medical people” without focusing on her own problems which might be triggering her
making sure she isn’t deficient in vitamins or minerals. Can affect mood.
encouraging her to take up yoga or something similar, and get in a very calm place before she is going to make a call.
With panic attacks as I’m sure you know, the thing that makes you most anxious is knowing you are going to panic and knowing how horrible that feeling is, so you do anything to avoid it, even if it’s quite irrational (like ignoring your medical needs). Avoidant and escapist behaviour doesn’t help in the long run, and it is very habit-forming, but if she is having panic attacks then she is in a really bad place. How is she with phone calls generally - is it only the GP that causes the anxiety? Does she live with you? If yes, then I would suggest that you involve her in lots of “official” calls - phoning up to check opening times of a shop, order a new bank card, making an enquiry about holiday insurance cover. You should do the GP calls for her for now, but she should stay in the room while you do the speaking with the call on loudspeaker. Schedule the calls so she knows when it will happen and she cannot avoid it. Get her in a calm place for the call - maybe right after a yoga or meditation. After a few times with you in charge, she should dial the number then immediately you take over the speaking. And next time she should dial and then pick the options on the answer service, then you take over. When she’s ready to try the call herself, make sure she has a basic script in her head of what she will say when she starts the phone conversation- she can practise this out loud with you.
Build her up slowly in other words, you want her to feel she can cope with whatever the call throws at her, so give her some stock phrases to use, for example “thanks for that information but I need to think about it so I will hang up the call now and call back another time.”
This isn’t babying her, it’s training her, helping her form new habits and push through/ handle the discomfort she feels in certain situations. You can acknowledge that she will feel some discomfort and fear but that exposure to that fear will gradually help it reduce. Like sitting in the sun - you don’t immediately sit out in the midday sun for several hours, you acclimatise your skin and get a light tan going, otherwise you’ll just burn to a crisp, and it will hurt, and the next sunny day you’ll be hugging shadows in a long sleeve shirt.
I’m sorry the cbt didn’t help and I also wonder if finding a support group might help. Something like TheTribe or 7 cups of Tea, online.