Which version are you using 3 or 4?
With 3 it's easy to setup. 4 seems to be a little more work (still trying to get it myself) but doable. And I'm sure it will be better with the real release of 4.
What OS?
Did you check your firewall ports?
Did you put in all the correct nV registration info?
What is your inbound rule for that line? Did you point it to an extension that is registered and working or did you point it to DR?
Deerfield support should really be helping you since you paid for it from them. 3CX and nV work fine together - I have several clients running both with no issues at all.