I have the same setup (D-Link DIR-655, 3CX V11 SP1, and Callcentric). No special settings required with the DIR-655. It automatically port forwards and has "smart" packet distribution. The factory settings should be good enough. I would check two thing: 1) make sure you set up the Callcentric VoIP provider correctly and 2) make sure you internet connection jives with your router setting (i.e. Dynamic IP for standard cable internet or PPPOE for DSL).
Also, once you get it working, if you don't set up static IP addresses for your machines, use the DHCP reservation list (setup>network settings). Consistently having the same IP addresses will save you grief in the future.
And update the firmware of the router if you haven't already.