How can Mum (living in same apartment) alert me in case of need, especially at night?

I use a baby video moniter.. I can hear and see my Mom in bed and if she gets up I can watch she's safe..

Refuses to, or can't ? Or doesn't understand the concept any longer? When you say " baby phone" do you mean baby monitor?

Can you talk a little about what the issue is? Is she trying to get up, do things that she shouldn't, and you tell her that she should call you? She may be at the stage where she no longer remembers that she needs to do that.

Might an alarmed bed bad be a better alert for you?