VOIP for business long distance and disaster recovery

I'm looking at implementing VOIP primarily for for outbound long distance, and as a backup to analog lines for business continuity and disaster recovery purposes. Travelers should be able to utilize the VOIP services as well.

I have two locations with 50 and 25 employees each in a town in rural northern California. We use AT&T Centrex and VirtualPBX.com for ACD. We've stayed with Centrex for the last twenty two years because of simplicity and dialtone no matter what.

For disaster recovery, we're implementing a satellite Internet solution from Skycasters which can handle VIOP. In a disaster, I want to be able to add VOIP DID phone numbers for each employee and PBX capability - ACD, voice mail, etc., quickly.

I like Skype’s structure with its ability to set up a corporate account for managing long distance funds and DID numbers for all users. I'd go with that, as a number of users are happy with the service, except that I need a softphone solution for Windows and Macintosh computers with a G.729 codec to minimize bandwidth utilization. Bria and Zoiper seem to be among the top programs.

Can anyone recommend a high-quality SIP provider with hosted PBX capability, and with this kind of centralized user management structure? It would be nice to goup users by division for easier billing. I don't want to pay per line until I need DID numbers as I'm not replacing analog service at this point. Just for outbound long distance charges for now.

I'm not too interested in putting in IP PBX hardware at each location, but have looked at a few vendors, like Rockbochs, but am open to reasons why it would be an advantage, bearing in mind that we're staying analog for basic service. The IP PBX boxes at the two locations would have to integrate with each other for common voice mail, call groups, etc. in the event of a disaster situation. Travellers would need VIOP access as well.

Thanks.

Tom