the admin guide really is the best way to do it.

one word of advice, do not use 911 as your testing number :). Use a 4 digit extension as the number you use to "dial" 911. Then when cucm goes to dial the 4 digit number, convert it to your cell number.

Basically, the end results, when you dial this 4 digit extension, it goes to CER, CER looks up the ERL zone, masks the number as the callback zone, pushes it to CUCM, CUCM then calls your cell phone. Caller ID will show the ERL zone the phone called from.

Once it's all tested from each zone, switch back to 911.

But it does sound like you need to read a little more to get a better understanding how CER works.