How to get mother to stop calling me at work?

I'm sorry for what is happening. It is a very common problem when our LOs are at the beginning of dementia. It will help you to watch some Teepa Snow videos on YouTube so you'll have a better understanding of how dementia is changing her and why it is changing her. You will also get very helpful guidance on how to interact with her for more peaceful and productive engagement together, You will need to look at her differently now... the Mom of your youth is being overtaken by her dementia and she can't help it.

My first response was going to be to just not answer your phone when she calls and let her calls go to your voicemail, but after seeing all the issues your mother has on top of dementia, it's probably best that she not be left home alone while you're at work and time for you to be looking into having her placed in the appropriate facility where she will receive the 24/7 care she needs and you can get back to just being her daughter and advocate.
She is only going to continue to get worse, as her brain is broken, so if you're wanting to continue working, it's best you get things lines up for your mother sooner than later.

You’re correct, your mom doesn’t understand and sadly, she never will. Are you sure it’s a good plan for her to be left alone? Many seniors with dementia wander away, or attempt to cook and accidentally cause a fire, or have a fall, or any number of other calamities can occur. I’m in no way blaming you, the situation is overwhelming for certain. But it doesn’t sound safe for mom. Consider if it’s time for another arrangement