It may be uncommon for most VoIP administrators to be interested in setting up services for multiple customers, but many of the issues that arise in this situation pertain to all voice systems. That said, many of the newer versions of IP PBXs and "softswitches" have some pretty neat features that allow you to provide telephony services to multiple organizations. However, you should think long and hard about this decision as there are a lot of compelling reasons not to do this.
At a glance, most of these features revolve around the ability for each organization to have their own dial plans that potentially overlap. For instance, you just might be an office complex landlord, providing phone service to several tenants in a big building. Each of your tenants could use 4-digit dialing, and each of them could use the same block of internal numbers, for instance 1000-1099. If a user in company A dials on extension 1001, then the IP PBX is smart enough to ring 1001 in company A instead of extension 1001 in companies B, C or D.
All of these features potentially allow you to save a lot of money by implementing one phone system, instead of one phone system per tenant. However, IP Telephony systems have inherited some of the legacy problems of data networks and some of these problems can substantially increase your day-to-day administration costs, turning a cost savings into a money pit.
Number one is security, of course. Fundamentally, maintaining a secure environment is a little more challenging with VoIP because you will need to allow voice traffic while preventing any other types of traffic from crossing between your customers. Also, different customers will have different security needs, and when using IP Telephony, sometimes those needs can be incompatible, which was almost never the case in traditional telephony environments.
Another aspect that is not to be overlooked is availability and change management. Unfortunately, some changes on IP-based switches still require reboots, or at least services to be restarted. Thus, an upgrade for one customer means outage for another. Finding a "good time" to make a change can be a real challenge when lots of customers are involved.
In the end, it still may be worth your while, but think about management while you're in the design phase and be sure to set your customer expectations realistically.
Thomas Alexander Lancaster IV is a consultant and author with over 10 years experience in the networking industry, focused on Internet infrastructure.