What version of Unity are you installing?
Here is the guide for 5.0 and all versions are similar for this integration however it is better to have the integration guide for that paticular version.
http://www.cisco.com/univercd/cc/td/doc/product/voice/c_unity/integuid/cxsecu5x.htm
Yes you would have a modem from the telco and a smdi (rs-232) cable. There is a diagram in this doc of how it is set up.
The Unity server will have voice boards for the analog.
so analog lines from the telco go to the voice boards in the server, and a rs-232 cable goes from the modem to the server.
Again what version of Unity are you installing?
rlp