Ideally, it is best to only have an Internet Modem that does not have a built-in Router. Then supply your own Router.
In my case, I have allocation of 2 dynamic IP address from my ISP through their Modem only (no router in Modem).
I then have a 5-port switch connected to the Modem output.
Then, on port-1 of switch I have a Linksys SPA2102-NA VoIP ATA with built-in NAT Router and QOS.
I have another SPA2102-NA connected to Port-2 of the switch.
I then have another Linksys WRT-54G router behind the SPA2102-NA. Then my PC's
I like using the SPA2102-NA's as the network routers because they automatically open all required SIP and RTP ports required - even for devices connected to it on my local network. It also enforces QOS. For example, my softphones on my PC behind the SPA2102-NA will work fine without having to do any extra port forwarding. However, any VoIP devices behind the WRT-54G router do require that the WRT54G be configured to properly port-forward or port-trigger SIP and RTP ports for any voip devices behind it.
If you don't want to use an SPA2102-NA after the Modem, then the WRT-54G can work fine. It is just a matter of configuring for Port Forwarding or Port Triggering on the necessary SIP and RTP ports. Then, ALL your IP-Phones behind the configured router should have access to the necessary ports. You shouldn't need to use DMZ.
If you have no choice but to use the ISP supplied Modem/Router combo, then you need the ISP supplied Modem/Router User Manual to explain how to set it for Port-Forwarding or Port Triggering on SIP and RTP ports. Once your Router is configured for Forwarding or Triggering of necessary ports, you should be able to connect as many IP-Phones as you like behind the router without any further fuss.CQVoIPhttp://www.cqvoip.net