N600 cable Modem with voip system

[servotsc]

I purchaced a n600 cable modem / router unit to swap out my cable company modem and simplify my network. Currently i have the cable co provised modem conencted to my voice system which needs an external ip and have it providing dhcp back to all my devices. Is there a way ic an have one port on the n600 act as if if is the external ip so my voip system will work? 

[DarrenM]

Hello servotsc 

I do not believe that you can just have one port on the Moden router do that but are you able to open ports or put the voip in the DMZ so it will work while connected to the N600?

DarrenM

[servotsc]

Wouldnt it be a security risk to put it on the dmz? Any way i had to return the modem my isp would not allow me to have that unit they say it was not supported.