Reminding probably won't work. If you have an answer machine could you turn it off so that unless you pick up there will be no connection and no call charge.
Of course you could get a more agitated and confused Mum because she cannot contact you. Maybe a laminated A4 sheet next to the phone telling her you are not available to take a call between certain times but this can be moved or ignored.
There probably is not an easy answer to the problem and the constant phoning may well pass and be replaced with yet another annoying and costly behaviour.
my mum is with Virgin and I had the call plan changed to include all numbers including mobiles as she was constantly making phone calls. I cant imagine a phone bill like that, I think it will be far easier to look at changing tariffs rather than get her to stop making phone calls, in my opinion. Good luck.
Not sure if this helps, but I am with Plusnet for phone and broadband. There is a credit limit of £55 or £75. As I'm new I can't post any links, I'll have to explain. As you make calls each reduces the credit. At billing time if you spend, say £20 on calls, your bill for calls is £20 and your credit limit renews. If you reach or go over the limit calls get directed to the Support team, who charge £5.76 to correct. Emergency calls can be made. You have to opt up the credit to carry on making calls (or not I suppose to not do so).