I have found a good program which can converse voice and video into media files. It is provided by Ozeki VoIP SIP SDK and with it you can play back the pre-recorded media file into a SIP call.
If you are interested in media file conversion technologies you will found more useful information on this site http://www.voip-sip-sdk.com/p_407-voip-audio-conversions-voip.html
. You can try it for free.