[Freeswitch-users] combining multiple SIP providers

Hi What is the best way to combine multiple SIP providers to achieve 1) Higher concurrency (for eg. 2 providers with 50 concurrent calling limits could be combined to give a limit of 100) 2) Redundancy (use another if one is down) I have a feeling that this will need some SIP Proxy like OpenSIPS but what could be the architecture ? Much thanks! regards, Anita