On any Mitel that I know of. if the DID does not have a valid termination point the system will reject the call. If you have an SX200 form 19 might route undefined DID's to a difined location. On the 3300/sx2000 it might follow an invalid routing point if difined.
Future reference, it's helpful to mention the type of PBX you have, type of trunking Pri, Analog, T1 and also tell us why you want to do this.