Lone Cedar, perhaps the reason you don’t have an answer is that the question is too vague. You need to state the starting point, i.e. what is in place now for voice, network, and handsets.
Two Asterisk systems connected over the Internet via IAX2 (plus the correct cards to connect to PTSN) and a LAN with Hard/Softphones is all you would need.
The only way to provide an answer is with the correct question, like: I have two call centers with 20 stations each currently using a Nortel Key system with T1 and Telco wiring, etc…