In order to customize and set up your voicemail, you must first enable voicemail in the "Settings" screen of iCall for the iPhone.
Once you have done this, simply click the "Voicemail" button on the tab bar of iCall (the bottom) and press "0"
Press:
1 to record your unavailable message. This is the message that is played if you have enabled voicemail, have not enabled forwarding, and do not answer an incoming iCall phone call.
3 to record your name.
4 to record your temporary (vacation) greeting. This greeting will override your normal unavailable message when set.