Well, I believe what you need to do can go straightforward if none of Asterisk or A2B decided to give you some hard time and get you to pull your hair few times. (they do that to me all the time)
1) you need to make sure that (trustrpid) and (sendrpid) are enabled in /etc/asterisk/sip.conf
2) (Auto set CLID) and (Force CLID) are both disabled in A2B agi-config
3) then you can define the desired callerID in the ATA device itself. 12 is not a big number to deal with manually ..
Also, make sure to set the 'callerid' parameter of the sip account to the correct value ..
callerid = Carlos <123456789>
Note : I am writing here from memory.. you might need to experiment a bit and read the supplied documentations to get everything in place ..
Hope this helps .. Keep me posted with the results.