Absolutely no need to apologise @BlueBay thats the whole reason I started this thread, so everyone can have a space to talk about their experiences. It's a space for all of us.
I completely understand that fear, and I'm of the mind that fear is a good emotion/sense to have. As long as it doesn't control us. Observe your fear, understand why it's present, accept it, but don't let it dictate what you will and won't do when it comes to your mother. Fear keeps us safe, but it can also get in the way when it goes into overdrive.
Unfortunately, I don't imagine your dad will be in a position to stand up to your mother. He has to live with her and find a way to make life for himself manageable, standing up to her would create chaos for him.
Is calling ahead absolutely necessary? Or is it a rule imposed upon you by your mother? No matter what she says or does, you have a right to see your Dad and he has a right to see you too. She doesn't get to decide that for him. Even if she digs in heels about it, you don't have to do as she commands. You and your father deserve to spend time together, especially if he is unwell. Keeping him from seeing his children is hardly going to benefit his health. It has nothing to do with his health and everything to do with her being able to control the people in her life. She doesn't have to keep controlling you. What you want is reasonable, your thoughts and feelings are valid, she does not have the right to disregard you. Try to accept she isn't going to change or do things differently, she is still going to have this controlling behaviour, but now that you're aware of it, you can choose to step out of her hold. She can only have power over you if you allow her to.
Again, I'm not saying it's easy to break the dynamic, and really, I get it. I really get it. They way she has and continues to behave isn't ok, it isn't fair, but you now have the power to respond differently to her. You just have to practice some self confidence, and trust that she has done wrong, not you.