Sure you can! Remember the VP number is really fake number created by Sorenson, so therefore VP number will not work between Sorenson and Dlink i2eye. In order to call somebody that have different brand of VP, you will need public IP address. Often I see people think 192.168.0.125 and that is a NO NO! It is NOT public IP address. it is private IP address. There is few ways to find out your current public IP address by visiting WhatIsMyIP.com - 22.214.171.124
Or find out though VP itself by click setting >>>> network >>>> public IP and the first line where it says auto-detect public IP address (XXX.XXX.XXX.XXX) that is the public IP address you need to give to your friend who have different brand of VP to call you.
The reason behind Sorenson's fake VP number is because public IP address can be changed from time to time, and no one likes to give away new IP address often, so the idea of VP number is to have a system that update the public ip address automatically and use it as reference with VP number. So that way, VP number stays the same EVEN when public IP address stays the same.
Finally when calling somebody using VP-100 to D-link i2eye, always choose ip address not VP number on the manual dialing screen. For VP-200, just enter the IP address on the dial box.