I have developed a similar app before. Users can set up reminders through a message to a bot on AIM, gchat, MSN ...
Then later they will receive a reminder message from the bot.
That was part of the plan. User send a message "8:30PM call me: You look handsome today." to the bot then later you will receive a call from the bot. It is like a command-line alarm on IM.
Yeah, I think in my case the "calling" part is the most important part. It's not even about the calling itself but more about the message. If I get a call from myself and it says "Hey man you have a test today in 3 hours, you can't be sleeping! Snap out of it!" that's pretty effective for me :)
Plus my favorite part is you can respond to yourself and it will be recorded--the dream capturing part. Please try it, would love to hear your feedback on these. Thanks!