Ours is from a local franchise of a national brand. Check out
http://www.esi-estech.com/ for their info. Their pricing isn't bad, but it's not a super cheap proposition, either.
We did need to purchase all new phones and you'll need a central phone hub -- forget it's technical name but it acts as the answering service, router, hold-music wherehouse, etc. -- basically the server for the phone system.