Well, the main problems with this approach are that:
1. an ACD agents should not be taking other non-ACD traffic
2. there should be no voicemail on an ACD agent
I tell you, #2 is a recipe for disaster in an inbound call center.
I suggest that you disable the voicemail completely, and/or use different extensions for the "basic" telephony and ACD (most hard- and soft-phones can handle multiple registrations at once), or you use "true" agents.