Couple of questions:
1. Is the destination under the receptionist Default operator a MM mailbox with extension?
2. Is it just an extension?
3. Is it a caller app association ID?
4. Is it a VDN destination?
If number 2 above,under the Receptionist tab do you have checked the two Transfer Invalid mailbox during and after business hours? What beginning digits under the Caller tab do you have checked?
If number 4 above, then I would ask what integration type as the VDN Operator has special considerations and is noted in the Config Notes for the integration type you are using.
Hope this helps.