I am thinking about 3CX for a call center with 250 Seats. There will be about 200 active users at any given time and at peak times 250 users with 400 more calls in Queue. Any advice?
I would recommend you go for an on-premise solution for such a high number of seats, you want full control over the system as well as the fact that for that many seats a hosted option would be challenging to deploy unless using a VPN or dedicated MPLS/VPLS connection (3CX SBC's at tops support 50 users per SBC on Windows and Linux).
For call center functionality also you would need a PRO or Enterprise licence and perhaps the possibility of using some of the solutions provided from some of the development partners (dependent on what features and functionality the customer wants) this way if there is something that 3CX cannot provide in-built for your call center you can get a possible bolt-on App to fulfill the requirement.