Different Telephone system types
I will try my best to help.
I will breaking a golden rule in doing so, as I never usually recommend a system until I understand the clients key drivers for change and their exact requirements.
Just a quick Intro Paul, Ive been advising client on what Telephone system or ACD to use for their business for over 10 years now and if you are Corgi in Basingstoke then I have worked with your predecessors. You initially purchased a GPT DXE 24/70 and expanded it and added a Storacall ACD to front the calls and to provide some queuing. The DXE suffered from a lack of ACD functionality and call queuing and rather than replace it at the time you added the Storacall unit. This was then replaced with the Meridian. From memory you had about 60 back office staff with about 30-40 agents. I won't ask why you are considering changing the present system as this can sometimes be for reasons other than technical or performance.
Systems fall into about 4 categories, The first are aimed at the SME Small to Medium Enterprise market, they have every feature imaginable for the office and usually come with some form of ACD so that they are not ruled out by clients who have a large central office with a small call centre, these include the likes of ( Panasonic, NEC, Toshiba, Ascom, Intertel, Samsung, Goldstar, Telrad,) and some of the larger manufacturers also have a product placement here (Mitel, Alcatel and AVAYA with IP Office and INDeX)
The second are Hybrid systems with strong networking features to link multiple sites and a versatile ACD which can cover most scenarios, offering real time reporting. A little bit more expensive but they offer a modular expansion path, better ACD and reporting features and the facility to add adjuncts such as Unified messaging, CTI, Diallers, IVR's etc. ( MITEL, ALCATEL, AVAYA INDeX, AVAYA MULTIVANTAGE-Used to be called DEFINITY, SIEMENS HICOM, ERICSON, INTERTEL etc)
The third are stand alone ACD's mainly aimed at those clients focused purely around call centre and contact centres, ( Aspect, Rockwell)
And the fourth are the new Breed of IP Solutions from ( AVAYA, CISCO, 3COM, NORTEL, SIEMENS, MITEL).
Let me know if you would like any comparisons on products as I have some competitor analysis documents. I also have some informative IP discussion documents and product comparisons if you are considering going the IP route.
Either way feel free to ask any questions.