What is it she wants you to do when she's having a panic attack? I suspect there is not much you can actually do. I'd look up some strategies for dealing with them and show her and then stick to not always taking her call.
I think some sort of home care would be a good shout as a PP mentioned. Otherwise I'd try to get her to find coping strategies that aren't you.
Also, did the GP give her any meds to cope in the meantime? Some people use Rescue Remedy and other homeopathic treatments. I'm not a big fan of homeopathy personally, but if you take her to a reputable shop, get someone to talk her through the options, and she picks whatever she prefers, then it means you've given her a way forward that's not calling you constantly while she waits for the referral to come through (I think some react badly with normal meds but you should be able to get advice).
There's also counselling via video calls or on the phone. If she's religious then various groups make services available online too. The big barrier might be getting her able to log in.
Also, would she do exercises at home? That'd help fill her time and hopefully make her feel more positive.
If she's refusing all reasonable suggestions then she's choosing to live like this, and you also have a choice.
Definitely make yourself unavailable on weekends.
Can other family members check in with her?
I think most people can stretch themselves on a temporary basis but if it goes on it does get harder, also if there's some sort of crisis (hopefully not) that takes away your time from her then things will crash very quickly... sometimes it does take a crisis to change things but you don't want it to be your own health.