It is quite common in the UK but not many providers call it "Centrex". It is more usually called a hosted telephone service, hosted PBX, virtual PBX, virtual switchboard etc.
http://www.coms.com/hosted-telephone-service.htmlhttp://www.telappliant.com/hosted-telephonyhttp://www.phonepresence.co.uk/One advantage of an IP-PBX is that internal calls connect directly whereas using IP Centrex it is likely the speech will go out to the hosting server then back in to the other extension - this is somewhat wasteful of WAN/Internet network bandwidth and could introduce delays/echo in the speech thereby affecting audio quality.
