Do other caregivers offer cell phone use to a 91-yr-old mod. dementia/memory care resident that lost her charger and forgets phone #'s?

No. Most memory care facilities help seniors reach out to a few family members a day or bring the phone to them to chat. It certainly would not be as constant as some today are with phone use, but it worked well for my brother's ex all the time he was in MC.

The landline isn't so great either, I hate to tell you. But the cell phone, fuggedaboutit 😑

When moms dementia progressed past the moderate stage, she'd forget to hang up the phone entirely! So I'd get a busy signal for hours and have to call the MC to ask a caregiver to go into her room to hang it up. She forgot how to dial too. Or she'd remember how to call me and say A LADY'S VOICE SAID I SHOULDN'T DIAL A 1 FIRST and all sorts of chaos. I started having to dye my hair twice a month for all the gray that phone caused me. It was a nightmare. I was getting ready to buy her a rotary dial phone with a cord but then she declined even further and stopped using the phone altogether.

Again, I love what you came up with for mom! Brilliant.

Wow that does look like amazing technology JoAnn and againx100!

I second the landline, and if possible get an old fashioned phone where you don't need to push any buttons to answer, just pick up the receiver and say hello.
(should be able to find one on Amazon)

I got mom what used to be called a jitterbug (now Lively) smart phone to replace her broken smart phone. OMG what a disaster! She has mild/moderate dementia and it was impossible for her to navigate. Couldn't remember to charge it. Couldn't remember to press the circle on the screen and then swipe to answer a call. Couldn't remember how to make a call. Yup, disaster.

Recently, I got her a new phone, using the service Telecalm. It's got a base you plug a landline into but using cell phone power. I have an app to control who calls her, who she calls, what hours she can use the phone, how many times she can call the same phone number within a certain time period, etc. I LOVE IT!

What about just having a good old fashioned landline phone put in her room? That way she won't have to remember numbers and can just answer it when someone calls.
Most cell phones are way too complicated for someone with dementia nowadays anyway. You've heard the saying that it's hard to teach an old dog a new trick right? Well it's even harder if not impossible to teach an old dog with a broken brain a new trick.