Should an 85-year-old with moderate dementia have a cell phone?

My mother spent about 3 weeks in behavorial health where she was diagnosed with moderate dementia and a personality disorder. She moved into a nursing home 5 minutes from me because her doctor said she needs to be in a skilled nursing facility. She also is partially confined to a wheelchair with peripheal neuropathy and only walks short distances with a walker and much assistance. Otherwise she is healthy with no body system issues (heart, lungs, etc. all in good shape).

She is fairly clear some days, mean as can be others. It's like flipping a light switch. She has always had a cell phone until I took it recently. She calls and leaves me very hurtful, nasty messages and this past Sunday she had a complete meltdown, threatening to call the police on me and her sitter. That's when I took the cell phone. She also tells others that we're trying to kill her and are stealing her money. Now she's asking for her phone and I don't know what to say. Her doctor's PA said to tell her she's too confused to have a phone right now but I know if I say that she will go balistic! I'm an only child and she has always intimidated me some, but now it's a lot worse. Every time I try to stand up to her about anything she becomes meaner to me and trashes me to anybody that will listen. I am seeing a therapist and she has told me to limit my visits to only twice and week and for only 5 minutes at the time. I go and talk to nurses and peep at her if I don't actually visit. Her sitter is wonderful and keeps me informed, although gives me a little too much information sometimes. Meant to add I am her POA. Thanks for any ideas.