To your first question, the basic differences between IP and digital for PBX systems are fairly well established now. The main difference is cost, where legacy PBX systems are mainly hardware-based, whereas IP versions are now mostly software-based. As a result, the PBX market has opened up beyond enterprises, and with IP, it is now affordable for SMBs.
Legacy systems are defined by their extensive feature set, but also being closed, proprietary systems developed by telecom vendors with a long history in this business. IP-based PBX systems have disrupted this market by offering a comparable feature set at lower cost. Not only that, but they offer a bridge into the broader world of IP, where voice is converged with data and video for a richer communications environment.
The world of IP is based on open systems and standards-based technologies, and as a result, many new entries are from outside the base of traditional telecom vendors. Aside from offering a broader range of choices for businesses to choose from, these vendors bring innovation and new ways of defining value for PBX systems.
Regarding your second question, there is no doubt that a small office of 30 could justify using an IP PBX in today's market. Prior to IP, a legacy PBX would have been cost prohibitive, and you would have had to accept a more limited feature set, along with a less reliable system and few options for upgrading. That is no longer the case, and there are many IP PBX systems available today, both from conventional telecom vendors -- who are now targeting SMBs as a new market now -- and next-gen vendors who have built IP PBXs from the ground up specifically for the needs of this market.
This was first published in March 2010