Check your hep file for "attendant overflow". Quick and dirty answer.
1. Make a rad (either use a vmail port or an ons rad)
2. Put rad in hunt group, type needs to be voice? (not 100% on that)
3. Reroute 1st alt the attendant LDN to the rad hunt group
Calls will still que at the console but the caller will hear the rad then hear MOH until answered.
I'm probably missing something in there, I haven't set overflow up in a very long time.