It sounds harsh, but I’d make sure the charger is ‘lost’, or it runs out of credit and isn’t topped up. Or it’s dropped down the loo… Anything really. It’s a common problem, where both the person with dementia and the relative they’re calling, are distressed by the constant calls.
Pretty much standard that once the phone stops working and is out of sight, it’s forgotten. As someone else has observed here recently, it’s a way for the PWD to put off transferring their care needs to their new home, and cling on to their former carer. Best for everyone if it just stops.
If you can’t bring yourself to do any of that, just block the number. Your Mum won’t remember that she’s repeatedly not getting through to you.