[Serusers] Support for multiple PSTN Gateways (hunting, crank back)

Hi all i have multiple gateway devices to PSTN. All are equal in terms of what destinations may be reached behind it or the termination costs. I use multiple devices for two reasons: capacity and failover. So what i need is functionality to hunt lines or at least doing a crank back (or even better both). The best way to this (IMHO) would be to track the line usage on each gateway and check it before routing to a gateway to make sure, there is a line available to place the call to (this would be a kind of line hunting). But i could also "live" with a solution that works more like a failover mechanism. In that case i would route the call to the first gateway in my list and if it is not available or has no more free lines, i would need to detect this, take the call back and place it on another gateway. As far as i could find out, there is no hunting, failover or crank back support directly built into SER. Is it possible to implement such functionality using the "common" routing scripts in SER? Did anyone of you ever had similar requirements and did you find a way to fullfill it? I'm very thankfull for any idea, hint, snipplet, .... Thanks a lot for your help. Best Regards Frank