Nat Public IP to Private IP behind Sonicwall TZ210 and Cable Modem

I need to "map" a Public IP address to a phone system on the private network. The firewall is a Sonicwall TZ210 and the primary wan interface "X1" has a "many to one" nat policy applied to it for internet access of the lan computers. When I set up a standard "one to one" nat policy to nat the internal phone system to a second wan ip address the network traffic is not being forwarded correctly by the ISP's modem. The ISP technician says that their modem cannot distinguish between two ip addresses when they are coming from the same physical port. Somehow I need to map this second wan ip address to a second mac address coming out of this TZ210. Anybody ever do this before? When I try to apply the second wan ip to another port "x3" for example, I receive a "this subnet has already been used error" and it will not let me do that.