Which SIP Server to Choose?

I'm trying to make our company's SIP/VoIP recording service to divide the workload on 2 computers that record the incoming calls and i would need a some kind of server program to redirect the incoming calls (they come through SIP Gateway from VarPhonex). Both computers can accept several calls simultaneously (using DialDictate for recording) but not enough for our company's needs and creating another voip number for the other computer would be quite non-userfriendly solution. I'm new to these things and haven't tested any server softwares yet. Any suggestions? Thanks in advance!