Followme - Caller ID modification

I have followme setup where it tries my internal extension then goes to my cell phone after. I would like to modify the caller ID of the caller before it goes to my cell phone. Out of the different options available under Change External CID Configuration there are options but none that will work.

I find if modify caller id and put a 5 before the number I can get something like
011 56615551212 to my cell phone which I can decode to 6615551212 but it would allow me to know it is coming from the office. I would like a way to put that 5 in front of the callers number. I could hack things together but I would like a better way if anyone has a suggestion.

FreePBX 2.9.0rc1.3 Asterisk 1.8.x