What are the options to increase the ports? Does the Commplete 4000 only use the ISI.... cards?

CC4000 only support Multi-Tech ISI cards. To expand, all you need is add three more ISI cards which will give you a total of 32 ports. If you want to support V.90 connections, then you will need to use the ISIHP-4S cards and you will need to get ISDN BRI lines for the ISIHP-4S cards.