This goal has the following fine print specified by pjh:
My mother is so much happier that I have this goal. I have already called her more often since the goal started, than I did in my entire adult life before that.
I hate making phone calls. To anyone. She loves it when I phone. Tricky.
Now she gets a call regularly, and it's more normal for me to pick up the phone. Everybody wins.
With the Beeminder premium plans, I've configured a cap on the automagic safety buffer so that it's never more than a month between calls. And though I phone her earlier in the day, an 8pm deadline was confusing because sometimes I entered the datapoint after that and it screwed up the always-on-sunday calculations.
Bad son becomes clever son, thanks to Beeminder.