I did think about selling it as "We'll force you to call your Mom /some time/ in the week. You may not know when!"
Then I thought I'd want to exclude the middle of the night (don't want to give people heart attacks thinking somebody died). Then I thought of excluding the work day (who wants to sign up for a service that will randomly call them in the middle of meeting?). Finally I decided it would be easier to set a specific time.
Maybe you're right though. Maybe the angle of "we'll keep trying until we find a good time for both of you" would sell better.