That's not too surprising. For the Hitron modems, in Bridge mode you can get two IP addresses apparently, and it doesn't seem to matter which ports are used, from what I've seen.
If you still run into issues with your Voip phone, part of the problem might be the ALG SIP setting on the modem which you don't have access to. You could call tech support to flip it either on or off, what ever is the reverse of what it is. You might also have to look up the ports that are needed for port forwarding for that phone and set those in the modem.