FOR BASIC E911 INFO: https://support.sip.us/hc/en-us/articles/205335075-e911-Important-Information
One of the most asked questions our support team deals with is, "What if I only want to have a single Enhanced DID for e911 but also want each extension to have a unique outbound callerID?" On any other PBX, you would simply create an outbound route for calls to 911 and set the outbound callerID on that route to your enhanced number. Unfortunately, 3CX denies you the basic (and extremely useful) feature of being able to set outbound callerID based on route. Although this causes us much more pain than it should, worry not, for we have a solution to your problem.
To work around this, we are going to have to create some 'linked trunks' on your SIP.US account (shoot us an email at firstname.lastname@example.org and we will take care of the trunk creation). These trunks will share the rate plan with your current trunk/s and will not cost you anything. On this linked trunk, you will set the 'CID Override' to the Enhanced DID you wish to display when dialing 911.
Now, you will need to register 3CX to this new 911 trunk. Afterwards, create an outbound route that states any calls with the length of 3 digits that start with 9 are to be routed through this new trunk rather than through your main trunk. Annnnd voilà, now any calls to 911 will route out this special trunk and even if your 3CX is programmed to send individual outbound callerID based on extension, your trunk with SIP.US will overwrite the outbound callerID to the Enhanced DID.
For those of you with multiple locations running off a single PBX:
The process is the exact same except you will need an Enhanced DID per location. We will have to create a 'Linked Trunk' for each location and you will need to create and apply 'Extension Groups' in your outbound route on 3CX to handle which extensions belong to which location.
REMEMBER: DO NOT CALL 911 TO TEST THAT YOUR ROUTE IS WORKING CORRECTLY. YOU CAN DIAL 933 TO VERIFY THE CORRECT NUMBER IS BEING SENT AND ALSO VERIFY THE CORRECT ADDRESS IS ASSOCIATED WITH THE NUMBER.