P-Asserted-Identity

My 911 carrier requres that we use P-Asserted-Identity, and I need to have it send the Emergency caller ID. Depending on the extension this value can be different. What I used was this and it worked great but I can’t get the value to pull from the Emergency caller ID. I know that I have this set for 811 this is just for testing purpuses. If I manually change the ${CALLERID(ALL)} to a number it works great but I need to pull the number from the emergency caller ID. Or if someone knows a better way to do this that would be great. Also I was given this script from our carrier and I am not sure what the second line of this does can someone let me know.

exten => 811,1,SipAddHeader(P-Asserted-Identity:) exten => 811,n,Dial(SIP/[email protected]_1)

Thanks,
Jon