When you create an overlay it will remain assigned for a period of time even if it is not called and for even longer if it is called. If an overlay expires, the number is still allocated to the destination and calls will be put through if it is dialled. The allocation is only broken when a new overlay request reuses an expired number and points it to a new destination. This system allows the highest level of availability for numbers and maximises the chance that the overlaying number for a destination will not change. However, in some circumstances it is advantageous to cycle through overlaying numbers more swiftly. These parameters allow you to control this process.