Modern Avaya and Siemens phones have about the same compatability as Nortel systems. You can pick up all three companies system’s phones from aftermarket suppliers at significantly reduced prices. A caution here is that having a mix of vendor supplied and aftermarket supplied phones (or curcuit packs or blades) can cause maintenance billing issues.
Be aware that phones from older or different systems might be re-usable on newer systems, but will not have all the same functionality or user interface as current phones. This holds true for all the manufacturers. None have a truly universal compatability with the possible exception of single line analog phones.
Find out who does the remote maintenance. It is not always the same vendor who sells you the system. Sometimes a vendor will use another company to do the remote maintenance. Not necessarily a bad thing, but now exactly how it will be handled. Who does the guy who comes to your office call to resolve a problem on the box?
Almost everyone can start as a standalone and incorporate voip, but I would still argue against a pure voip system as it would subject you to sever reliability. Saying that if you have a problem it is probably the server does not eliminate the problem. A purpose built phone system is still much more reliable. Besides functionality and reduced manufacturing costs, all the manufacturers are pushing voip as a way to get existing customers to buy something new. It was very common for companies to have a seven or ten year plan to replace phone systems, and the manufacturers have to overcome that. Imagine that you are a refrigerator salesman, once you sell one to a customer he won’t need another one for years - unless you can come up with something new.
If you are considering leasing a system, you should think about matching the lease length to that of your real estate lease. If you may move in five years, for example, you might want to simply get a new system at the new site instead of a “hot cut” and trying to move the old system over a weekend. Some people used to lease the system but buy the phones outright. Be sure to have spares - built in spare ports for lines and extensions, space for expansion, and extra telephones. You would be surprized how many phones disappear when a new system is installed. (Most of the multi-line phones won’t work at home, but people don’t realize that). Find out what training is included with the purchase (adminstration training, end -user training) and how is it delivered - a class on site, off site, DVD or on the internet. Find out about documentation - documentation for new systems is usually not free.
Having worked with Avaya, Nortel, and Siemens, I would say that all three are about the same as far as programming and set up. I never saw any problem with setting up any of the three systems. A common sales tactic is for one vendor to say the other’s system is difficult to administer or set up or whatever. My personal experience is that they all work equally well. You can have problems going from say a Nortel system to an Avaya or vice versa, because some of the terminology or procedures are different and have to be learned new. That does not mean they are difficult or confusing in and of themselves. Avaya’s advantage is functionality, Nortel’s is price, and Siemens is the name. Avaya used to be AT&T and Lucent, it was considered the “safest” choice. Nortel was the safest choice if you didn’t want to buy AT&T. Siemens was the safest choice if you were a German company (almost the only choice then) or one that did a lot of international business.
Find out just what kind of functionality you need especially in the call center - not all do things exactly the same way. You might be looking for a measurement or function that only one or two vendors can provide. This is the area where you will have to do more homework than any other.
Once you pass Avaya, Nortel and Siemens you are in what we used to call second tier territory. Here the systems can be just as good, usually cost less, but you are much more reliant on the individual vendor. A good relationship with a vendor you trust is probably the most important criteria in choosing a system. Don’t e afraid to shop around and ask for bids from multiple vendors, just make sure they are always for the exact same thing (number of phones, lines, functionality). A good salesman can design a good system for you, just make him do his job.