The 911 regs vary from state to state, but typically you should provide a unique calling number from each building that will resolve in the 911 database to the correct address. A callback to that number should ring a phone in that building. If your PSTN service is via T1 PRI you can set up the Call Manager to send a unique calling number per building. If you have POTS service to the building you might be able to use that number, as it will already be in the database. The way I have done this is to configure a set of Calling Search Spaces for each location, and use the route pattern config to set the calling number. A couple carriers gave us some initial grief over allowing our calling numbers to propagate to the PSAP on a 911 call, but we have been able to do this in Alaska and Washington state. We usually have 6-10 locations sharing a common PRI PSTN connection.
Here is a NENA link describing this type of setup:
http://www.nena9-1-1.org/9-1-1TechStandards/TechInfoDocs/Trunking%20for%20Private%20Switch%209-1-1%20Service%204-11-2003.pdf
Here is an excerpt from the Illinois E911 regs:
http://www.icommconsulting.com/Articles/E911CaseStudy.htm
In an E911-enabled community, both businesses and the respective PSAP send and receive additional and specific location information. The Illinois E911 Law requires one unique DLI per 40,000 square feet of workspace and (at least) one unique ten-digit number (ANI) that corresponds to that DLI. If multiple buildings under 40,000 square feet share one common address, a unique ANI and DLI per building location must be defined. For example, an 80,000 square feet building needs to send two ANIs, and the two ANIs must correspond to two unique DLIs.
If there are three buildings of 60,000 square feet each that are networked and share a common address, two ANIs and two DLIs per building must be defined. All phones in a 40,000 square feet “pod” can share one ANI with one DLI, and if the ANI is called by the PSAP 911 Operator, only one phone in the “pod” needs to ring, according to the Illinois E911 Law. See “E-911 Call Flow Technical Diagram” on page 10 for hardware, software, network services and infrastructure components.
