I assume you are using CUBE in media flow-through mode If so, if you aren't going to use HSRP, then you'll never get the calls to be preserved during an outage. However, new calls could be sent to the backup CUBE simply by having the provider configured to send calls to the other CUBE IP when the first is down. And on the CM side, you'd have another RG in the RL for the backup CUBE, so that outbound calls hunt to that guy when the first is unresponsive.
If you did flow-around mode, media wouldn't go through CUBE, so you could achieve call survivability on active calls if your provider disabled session refresh for the trunks. Because CUBE is stateful, when the original CUBE goes down, call control goes away and session refresh wouldn't be able to be sent from the backup CUBE. Even the box-to-box redundancy feature on CUBE with HSRP can't handle session refresh, though, since the failover isn't stateful (yet).
