For someone in need of a new business telephone system, it would certainly seem logical that if there were more than one separate organization involved - if, for example, a landlord was providing communications for multiple tenants - an independent phone system solution would be required for each company.
That is not necessarily the case, though. I am currently working on a project for a multi-tenant medical facility in which all of the tenants are controlled by their affiliated hospital. As such, the hospital is responsible for accomodating each tenant's communications needs (telephone lines, business phone system, voicemail, etc.). Rather than forcing the hospital to acquire separate and disparate phone systems for each of their practices, I was able to easily design a solution which takes advantage of the capabilities of a single system to meet the needs of all tenants concurrently.
There is a great deal of flexibility in the way that telephone systems can be programmed. Because each individual telephone line coming into a system can be told to route its calls to a completely different location than those of another telephone line, this allows the freedom to create separate groups of extensions, with each group representing a completely different company. Just because there are 9 telephone lines (for example) feeding the phone system, that does not mean that they are all accesible by every extension on the system. Similarly, a centralized voicemail system can be accessible by any extension on the same system, so that any of the companies (or groups of extensions, from the phone system's perspective) can have access, and as long as there are enough mailboxes to cover all of the extensions, no additional voicemail is required.
Returning to my medical facility scenario, I was able to use the exact model that I just described. There are to be three separate medical practices all inhabiting the same geographical area of a building, each with three telephone lines of their own, and roughly 10 extensions...so I will provide them with a single, centrally located, PBX telephone system, with ONE internal voicemail card. So long as I equip the system with the capacity to accept 9 telephone lines, 30 telephones, and enough mailboxes in the voicemail for all users, the rest will be left up to our expert technicians! They just need to make sure that the telephone lines for each practice are separated from one another and that those lines will access ONLY those extensions corresponding to the same practice.
So, if you are a landlord, a property manager, or even just have a neighbor that you want to share a phone system with, it may be a lot easier than you thought.