Probably the most important for the success of a call center is capacity. You need to be able to support the volume of calls coming in. You will also need a powerful enough call processor to take the calls, provide directory services and auto-attendant features and eventually relay the incoming calls to the right people.
One way to determine the capacity that would be required is to use the Erlang tables. see
http://en.wikipedia.org/wiki/Erlang_unit for more details on these tables and how to asses call volume and capacity.
I hope this answers your question at least in part.
Regards
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Miky Dinescu
www.digitalgetto.com