Thats what the dialer should ask for. Just use an analog a digital playback available at RAdioshack, up to 30 seconds of playback. Run you microcontroller on a clock program that waits xseconds and then dials a randomly generated pnhone number, or from a large list (may require seperate EPROM for library). Have it linked to the dialer of any phone (you will have to research the frequencies to send), or build an arm device to punch the numbers using servos and hydrolics. Its mostly about programing, and you will have to learn it through trial and error. Just remeber that you need to assign the pins to a single function, and use the I/O ports wisely, you may only need 12 or 14 outputs, but only 2 input if you dont uese the arm.