Honestly, from the perspective of an engineer for a business VoIP provider, you don't want to host the PBX on your end. We have numerous customers running Fonality, homegrown Asterisk boxes, etc - most of them eventually wash out and get either a hosted solution or go back to analog. If your business isn't telecom, you don't want to be managing that infrastructure. For large companies with Cisco CallManager it might make sense to host premise based, but that is because it is economical to have a full-time telecom person. Without a doubt, if you are looking for VoIP with analog failover you are best still going hosted and then doing a premise based analog backup (ie. using a Cisco IAD).