OP, I have the same problem with my mom, although her loneliness stems from the fact she's home and my dad is in a nursing home, permanently.
My mom will also judge/become angry if I don't call her often enough, but never calls me. It's maddening. I hate calling her because she just talks non-stop and doesn't take a breath. Ha! I got a call waiting when I was talking to her yesterday and I said, "Oh! Call coming in! Hang on!" Clicked over, chatted for a minute with my DH, came back and she was STILL talking and did not know I had left the convo!
My quick suggestion is to call her telling her you only have a few minutes and right away ask her if this is a good time because before you forget/have to go, you have to tell her ____.
If I don't just immediately bring up what I want or need to say, my mom will just start talking.