The SLB introduces a Peer Overload Control functionality. A configurable "neverRetry" option is provided in the system/SLB configuration for regular calls rejected due to congestion. Depending on the Global Loss Ratio, Multiplied Loss Ratio, and the "neverRetry" flag decision to retry the call, a drop or 503 relay is made.
On the SBC Main screen go to All > System > SLB > Overload Control Options.