I had this an ended up going NC for many, many other reasons too. Which is a bit of an extreme solution but I'd had years of it and, well, it started off with melodrama when they were in their 60s... if you didn't pay her sufficient attention you were "uncaring". With hindsight I wish I'd put down boundaries earlier, but by the time I did I already felt run ragged.
I remember once I had very high blood pressure and had to go to the docs and ended up cancelling the appointment because I'd had another ruddy drama filled phone call just beforehand about DF being about to die, that was a few years back and he's still with us. It did make me realise it was ridiculous that their health problems were taking precedence over mine though. And I can't even remember what the drama was.
I'd try and get the calls down to a couple of times a week. Have times - like morning, later on at night and at work - where you tell her you're unavailable so not to call.
You don't HAVE to answer if you don't want to. If there is something wrong, believe me, you'll know!
I also stopped my voicemail so she couldn't leave irritating lengthy messages.
It's ok to use a bit of gentle humour with the gasping, behaving like he's going to die imminently etc. Or say something about feeling worn out.
You could have another 20 years+ of this so try and get it down to a more manageable level now.