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.

Even just making it random around a certain time (say +-15 minutes). Another idea might be to have time blocks that a user and their mother can choose and then pick a time out at random from the overlap. Neither will know when they are going to be called, but it will be at a time that suits both.

You could of course go the other way and just do this without the user choosing anytime at all. When they register to send their mother flowers record their number and their mother’s from the delivery details and then just set up the “blind date” without letting them know. Each will think the other called them first.