Motivation:

You are an FMC (Fixed Mobile Convergence) cellular operator. That means you can provide both circuit switch services and IP based services (In English: Your users can make and receive phone calls via either the circuit or the IP (IMS) domain). You want incoming calls to your users to be routed either via the circuit or IP domain based on the user's preferences. However if one of the domains is exhausted (resource-wise) you rather route incoming calls to the user(s) via the other domain rather than fail them.

The feature:

Assuming Policy and Charging Control (PCC) is deployed in your IMS/FMC network (you better make sure it is!). Upon an incoming call, the IMS server (P-CSCF) consults w/ the PCC Policy and Charging Rules Function (PCRF). The PCRF lets the IMS server know that the call can not be completed via the IP domain (resources are exhausted), and it better select an alternative route (the circuit route in this case).

How this can be accomplished:

Figure 1 below illustrates how this can be accomplished.