Your mother has clearly engaged in this behavior for a long time, probably decades. As most people know, yelling is a very unproductive and ineffective way to parent and provides a poor model of self-control for kids. It is certainly not unusual for parents to raise their voice and yell at their kids. If this doesn’t happen frequently and the parent apologizes to the child for their inappropriate behavior afterward, no lasting damage is likely to be done.
It is extremely inappropriate for your mother to be screaming at you or for that matter, at your children or your spouse. Many people handle this situation by ignoring it and saying “That’s just the way Mom (or Grandma) is. Just don’t pay attention to her.” Avoiding conflict certainly seems easier than confronting it. You may even have tried to confront the issue in the past and had it do no good.
Certainly a first step is to try and address the issue with her, preferably face-to-face, and let her know that you will no longer accept this behavior from her. It’s best to meet either on neutral ground (like a park or coffee shop) or at her home so that you retain the option to leave.
Keep the conversation short and to the point. Ask her not to do this to you or to anyone in your family and let her know that in the future if she continues to raise her voice to you on the phone, you will hang up and talk to her at another time when she can speak to you calmly and in a normal tone of voice.
If she starts yelling at you as you try to bring this up, give her a verbal warning that if she continues, you will leave. Then give yourself permission to leave if she doesn’t regain her self-control after two warnings. It’s OK to protect yourself but be careful that you don’t do this in a rage or in an aggressive manner. You don’t want to be like her; you just need to set a limit on how she can treat you. Let her know that you can’t continue talking to her right now, you are going to leave and that you will talk another time. Then leave. Don’t try to argue or convince her. At this point doing so is futile.
Follow the same process if she begins to scream at you on the phone. Give her a couple of verbal warnings that you will end the conversation unless she stops yelling and if she continues, follow through and end it. It takes two people to engage in the dynamic: the yell-er and the yell-ee. If you stop being the yell-ee she no longer has you to yell at. Her behavior may or may not change but you do have the power to change yours. And it just might lead to a significant change for the better in your relationship with her.