Theoretically, the patent office is only supposed to award patents for "non-obvious" patents, and the concept of converting between an IP address and a phone number certainly seems obvious.
Unfortunately, our patent system has long since departed from this ideal. In recent decades, the courts have dramatically lowered the bar for obviousness. As a result, some of the patents being granted are so broad that inventing around them is practically impossible. The patents that allowed NTP to win a $612 million settlement from BlackBerry maker Research in Motion, for example, essentially covered the concept of transmitting new email notifications wirelessly. There's no way RIM could have "invented around" that.
If we are going to have patent reform and a patent arms race: how about at least allowing Vonage to countersue for abusing the patent system through overly broad and obvious claims?