A2billing Cluster - Asterisk/Freeswitch, OpenSIPS/OpenSER

We currently have a single A2billing server with a few clients on it. We are expecting the numbers to grow soon. We are interested in knowing more about creating a multi-server cluster using OpenSips/OpenSer to redirect the calls like a routeur to different server in fonction off the amount off calls on the servers or the cli off the calls or hours the time off day to different sip server like Asterisk/Freeswitch. The cluster should have a single instance of A2billing so that we can manage everything on one interface but the calls should be load balanced over multiple servers. We are currently capable of doing over 400 max simultaneous calls with long duration calls on a single A2billing server. We need to be able to load balance the calls from the OpenSip server to different SIP Server where the calls will be redirected Our target figures are 1000 simultaneous calls of Short duration calls , so we would need whatever can give us that. The setup needs to be expansible, so say few months down the line if we need more capacity we should be able to add more servers to increase the overall capacity and thus number of simultaneous calls over GUI . The right candidate should have multiple experiences setting up a similar cluster. If possible should be able to show us the work and provide references of current or past customers for whom you did a similar setup.