http://the-asterisk-book.com/1.6/einfache-ivr.html

https://wiki.asterisk.org/wiki/display/AST/Creating+a+Simple+IVR+Menu

http://stackoverflow.com/questions/6770737/asterisk-create-a-multi-level-ivr-example

http://www.apstel.com/asterisk-voip-how-to-make-ivr/

http://www.digium.com/en/products/switchvox/support/a-basic-ivr