I have a school district where we use 4000, 5000,6000 and 7000. 4000-4150 is one school 4151- 4250 is another and so on. The indiviual blocks Identify individual schools. For DR and redundancy each school has it's own phone system all networked all connected with fiber. They didn't want one large system in the high school and IP handsets to the other schools for security and bandwith concerns. They didn't want hosted for the same reason. Everything works exactly how they need it to be. We also created 1 instead of 9 as there outbound digit and a table so they can just dial 911 or 1911.
