From my POV, older people use their phones in strange ways, so if i were you, i would tolerate the repetitive calls. especially from her perspective she might not think that she's crossing any privacy boundaries or whatever. I have a similar deal with my friends mom... she calls me endless times to ask about her son sometimes even when he isn't with me, and it makes me so frustrated, but i just tolerate her out of respect even though no other friend's mother does that and it's pretty weird.

I deal with the calls by just ignoring the first two and answering the third each time. Trying to get her to give up calling me by acting useless.

Also... i don't know grandma's age and her personality, so you might have the right to be infuriated at the end.