First, the following assumes Intuity Audix:
Depends on how your call transfer out of audix features are set up. I'll tell you how mine works and you can play from there.
'cha sys feat' page 3.
Transfer Type = enhanced_no_cover_0
Transfer Restriction = subscribers
Covering Extension = xxxx where xxxx is the extension assigned to your receptionist.
If the receptionist is assigned as an attendent console you need to 'cha att x' and verify there is also an extension assigned to plug into Covering Extension. In this scenario you also need to create a mailbox for your covering extension if one does not exist as Audix will only transfer to administered mailboxes. I made mine an announcement mailbox with no announcement as my console is manned 24-7 365. You'll also want to log into the mailbox and change the password and record the name if you have names set to required (even if you don't it's a good idea to record 'reception' or some such so the caller doesn't get confused).
Also, on the subscriber form there is a field for Covering Extension. If for some reason you want a particular individual's phone to go to a different extension you can specify it there. I have multiple facilities with different receptionists off the same Audix so this comes in handy there it would also work if you wanted the CEO's mailbox to ring a particular assistant.