How to have a phone conversation with someone who has significant hearing loss and dementia?

My mother is 98 years old and in a nursing home. She has lost most of her hearing and stopped wearing her hearing aids several years ago. She has extensive cognitive impairment.
My brother visits her twice a week, and thankfully she still recognizes him. I live 300 miles away but, at age (75), health problems prevent my traveling anymore to visit her. I call her about every 4-5 days and am always relieved that she still recognizes my voice, but that's about the only good thing I can say about these phone calls. Because she can't hear very well, she doesn't understand what I'm saying, no matter how slowly I speak or how carefully I try to enunciate my words, and I do try to keep the sentences short and simple. I'll say something and she, in trying to understand what I just said, repeats it back but it's hopelessly garbled.
It seems safer to let her lead the conversation, but mostly what she says is confused, and many words aren't real words at all. I listen intently, trying to identify her mood, and then offer some reply ("Oh, yes, Mom, you're so right." or "Yeah, Mom, I know, you just can't get everything done in one day." And so on.)
This goes on for about 20-30 minutes. I feel her confusion and frustration when she doesn't understand what I've said, and I feel my own frustration when I can't figure out what she's trying to tell me (or how much of it is real). Sometimes I think I should call her a lot less often, to minimize her struggle with phone conversations, but then I fear she'll think I've forgotten her or, worse yet, she'll forget who I am and won't recognize me/my voice when I do call.
I'll add that I do send her a notecard every week, something I started doing a few years ago when my brother and SiL moved her into their house. Back then she looked forward to the cards and the short notes describing what my husband and I were doing, and asking her about her life, her bird feeder, etc. As the years passed she grew vaguer about them, not sure if she'd received one that week (my brother always assured me that she had), couldn't remember what it looked like, or what I'd written. Now, in the nursing home, she doesn't mention them at all. That makes me sad because I'd like to think that a pretty card every week would brighten her day, but they seem not to register with her at all anymore. My brother does think that just the act of an aide bringing her something for herself cheers her up, even if she promptly forgets what it is, and so I continue to send a card every Tuesday.
Does anyone have any advice on how I could better talk with my mother on the phone, or anything else I could do to maintain the waning connection between us?
Thanks for any advice or insights!