Cisco VoIP integration with PBX system

I have an existing PBX system and would like to convert it using Cisco VoIP.I’m suscribing a 1Mb leased line from the ISP and my router will be Cisco2620 with an VIC-2FXO card installed.Do I need a Cisco CallManager to get involved in order to get at least 10-20 call simultaneously at one time.I’m planning to connect the existing PBX system to my router and all the phones will able to use the leased line for calling.
What is a simple hardware requirement?,I knew that VIC-2FXO only allow 2 callone at a time if connected direcly to the PBX system.
PLs advise.