I am developing a call center which is necessary for my business communication. I am using this sample http://www.voip-sip-sdk.com/p_419-voip-call-center-server-voip.html
provided by Ozeki VoIP SIP SDK. It is operating with an IVR system and it navigates my customers through the tree-structure menu. It can transfer an incoming call to my human operators as well. With this SDK you can easily develop a simple call center server.