[Users] Distribution Calls with the same destination number to different servers

Hi all, I am working on the following scenario: Our carrier sends all calls (SIP protocol) to my openser, which forwards the calls depending on their destination number to four asterisk servers (for load sharing reason - we're speaking of a roof of 300 concurrent calls). I have one number which leads to a conferencence (one speaker, multiple listeners), whoose number of callers will exceed the number of calls a single asterisk will hold. Therefore I want the openser to distribute the calls to this number to all four asterisk servers (to share the load). My question is: how do I configure this on openser, as I have to send all requests from the same SIP session to the same asterisk? Can anyone give me a hint on this? Thanks in advance. Andreas