When you attempt the transfer to voicemail are you using the '*' or '#' key? Does it work if you do it from the phone vs the Attendant Console? What is the CSS of the gateway? Perhaps the gateway CSS does not have the partition for the voice mail ...
You could use time-of-day partitions/calling search space and route to the lead number of the hunt list during the day, and voice mail in the off hours. Then the receptionist wouldn't have to do anything.
Force authorization codes came out in version 4.1. I suppose you could use translations patterns and carefully planned partitions and calling search spaces to accomplish what you want.
Interestingly, I just had this conversation over lunch. My experience has been that even though the FXO card theroretically only needs 4 channels, it allocates all resources on that 16 channel DSP SIMM. You will need a second DSP SIMM to allocate cha...