N.B.
I used to take my mom to the doc. He'd talk to me at her appointment and she'd get mad. She'd tell him she was "over here" but the doc understood that she wasn't capable of taking care of herself.
I think you can ask grandma if she'd like you to handle it for her then have her talk to the company first, telling them she'd like them to freely speak with you, for this one phone call, and see if you can handle this.
I'd offer to be her advocate only when it seems she needs help but tread lightly. She's likely feeling old and worn out and doesn't need someone coming in saying she's not capable. So speak to her in a way to say you might be able to get them to understand there's a serious issue but not make her feel smaller and older and less useful.