The carrier should tell you that info.
But, to answer your question, there is a way to do that.
Edit the form "intercept handling". Use instance #2.
In the fields for invalid dialing put in the number you want to route it to.
Next in the trunk attributes for assign '2' in the "intercept number" field of your PRI or SIP trunks.
Now if a number comes in that is unassigned it will ring to where ever you pointed it to.
Ralph