Ok, so the E911 is a sip only thing for the NEC CP20 (only). If you are on a PRI the carrier is where you need to start this. You can send CID...now if the carrier can send the proper address (in their equipment) then you are gtg. I am fairly certain Comcast can do this. Otherwise you have to get a SIP trunk (1 or 2 should do) and route your 911 calls to them.