if 90 days, with freeswitch conference server+ any customized softphone ,we can build such a system,if you need crsytal and stable voice quality,I believe gips voice engine(which is wide used in skype,qq,maybe MSN voice )
sure we have many to improve/test if given 1 year or more .
I am voip professional, proficient in asterisk,c/c++,mysql, I am also good at php,flex ,website ,drupal ,linux,mysql,javascript, etc
I also have good experience to build one large scale predict dialer and hostable callcenter
freeswitch running on linux,you can find many open source softphone(to be improved or customized) which can running on windows,linux,mac,etc