You should be good to go. However this isn't the best configuration. I think you are bound by your ISPs HW configurations so this maybe the best alternative that works for you.
I have a similar VoIP device however my ISP modem is a stand alone mode thus NAT is solely handled with any router I connect to it. So with my VoIP device, I connect it to the router and put it in the DMZ so there isn't any NAT or un-necessary processing issue on the VoIP device and it's connected clients. I had tried putting the VoIP device before the router however considering that it only supports 100Mb on the LAN connections where the ISP modem and routers support 1000Mb connections, I didn't want to put a bottle neck in the mix. This needs to be taken into consideration as well when using VoIP devices.