The integration of Unity and the Avaya G3 does not require a serial cable from the switch. They can do an inband dtmf integration using analog ports if the switch supports VMI Mode codes, or they can use a Calista PBX Link.
The PBX Link emualtes a digital phone and only requires a digital station port from the switch to the PBX Link and then their is a serial cable that goes from the PBX Link to the Unity server. The digital station plugged in to the PBX Link is programmed to have bridged call appearances of the analog ports that are plugged in to Unity. When a user calls an analog VM port, the PBX Link sniffs the display information off of the bridged call appearance and then wraps that up into a serial (SMDI) packett and sends it to Unity.
See the following link for more information about the two diffrent integrations Unity supports with the Avaya G3.
http://www.cisco.com/univercd/cc/td/doc/product/voice/c_unity/unity31/integuid/index.htm