Call Transfer


this is what i would like Asterisk to do. i have call transfer feature on my VOIP phone service. I have that phone connected to my Asterisk. so i would like to call my asterisk and have asterisk answers the call and press flash or # and get a dial tone dial the other number i want to reach and asterisk hangs up and the two parties will be still connected.
i know it can be done but i don’t how to do it
please help

thanx in advance