OP, I think I may be in a similar position to your Mother, as I have severe mobility problems, and I do feel lonely at times, but try not to put pressure on my family to visit, and when I am able to travel, I make the effort to go and see them.
I think from what you've said, that you need to perhaps write your Mum a letter, pointing out how much you actually do for her, how much you have to do for yourself, ie, work, family, home, etc., and end by saying, that you simply CANNOT DO ANY MORE, and so if she continues with guilt tripping you, and hanging up the phone when you call, if she can't have her own way, then you will be cutting back on your visits, and calls, as they're obviously not appreciated. Remind her if you like, about the fact that after falling out with her own mother, she stepped back, and left caring for them to YOU, so she really doesn't understand how much you have going on. Then see what happens, ie., if she hangs up on you again, you cut down your calls from every day to every other day. If she objects to you taking a weekend off when you need one, you start going every other week. Then if she continues to gripe about it, tell her that rather than achieving what she wants with all her moaning and complaining, all she's actually succeeding in doing, is pushing you away, and she needs to think very carefully about that before she loses you altogether!
I know you'll struggle to do this, but she really can't demand so much of you. I'd be happy with a visit once a month from my family, but if they can't make that, then I'm just pleased to see them when they can come.
Often you'll find with parents that they will make it awkward when you need to leave, no matter how long your visit has been, which is what we experienced with my MIL. My own Mum on the other hand used to say, "I hate it when you have to go home, but if you don't go, I won't be able to look forward to seeing you next time", and would wave me off with a hug and a smile. All of which made us want to go more often, whereas when they say things like 'Oh, can't you stay a bit longer, it'll be ages before I see you again', and stuff like that, it just puts you off visiting, and you end up visiting them out of duty, rather than going because you actually WANT to spend time with them.
Good Luck!