With traditional Centrex there are typically multiple analog lines from the carrier installed in a business and analog phones are used. With IP Centrex, the multiple analog lines can be reduced to a single connection, such as a fiber optic interface running IP over Ethernet or a T1 circuit running IP over PPP.
As with traditional Centrex, IP Centrex service is hosted by the carrier. The carrier usually provides IP phones for the business. As IP Centrex is replacing traditional Centrex, IP PBXs are replacing traditional PBXs.
The next logical question is whether to use IP Centrex or an IP PBX. This really depends on the particular business. If a business comprises multiple small offices that are geographically dispersed and there is no IT staff, typically IP Centrex would be the best solution. If a business has IT staff, typically they want to have as much control as possible over a system. They usually choose to install an IP PBX at their business.