What you have now would also come into consideration. If you have a Siemens system it could be beneficial to look at the new systems from them (OpenOffice or HiPath 4000), likewise if you have NEC, or any other manufacturer it makes sense to look at them. Cisco is the big name in VoIP, but they are by no means the easiest to use, or the most cost effective. What you wrote you are looking for is supplied by most major manufacturers.

Steve