The latest CUBE release allows you to have locally redundant pairs of CUBE routers when using ISR G2 hardware. In this case you would have four routers, two at each location.
http://www.cisco.com/en/US/products/sw/voicesw/ps5640/products_configuration_example09186a0080b40d82.shtml
The next step up from that would be to use ASR hardware that can provide software/hardware redundancy in a single chassis.
A Cisco Unified SIP Proxy (CUSP) can provide CUBE load balancing but it does not address dialogs already redirected to an individual CUBE.