Hi,
he should have at least some docs about Networkplans/passwords/..etc. If he dont have, than you will need lot of time to figure out things, especially if you are new to IP-Telephony. I suggest you start reading about Cisco IP-Telephony. Also if your company can afford a test Envirement for you, so you can test things (CCM,Unity) before touching the Productive system.
Keywords to start with:
-Callmanager SRND(Design), BARS,RTMT(real time monitoring),ART(call detail record)
- Unity Administration, Backup:DIRT,EMS(Alerts)..etc
- reading through the Netpro forum will also help you a lot.
and last but not least, you need to stay cool :).
Hope this helps.
Karzazi