According to the website of the VoIP box vendor it is in fact a router.... And there is your problem: Instead of your computer having direct contact with your ISP, it is now talking to the VoIP box, which in turn is connected to the ISP.
So, I can't really guide you through setting up the VoIP box properly, but the general idea is to go through it's manual, and look for the entries on "port forwarding". First check the IP address that your computer has gotten now, I have no idea how to do that on a Mac though but it is probably 192.168.0.2. Then open a browser session to your VoIP box as instructed in the manual and forward 3 ports to the IP address of your computer: TCP (!!) 4662 and UDP (!!!) 4665 and 4672.
This should fix your problem.
PS: Oh, and remember, if you are unsure about your actions on the VoIP box, just don't hit "save", kill the browser, read the manual again, and some posts here on port forwarding, and just try again. Computers and routers can't die, they only can be misconfigured
