search for: set_cid

I am trying to find out how I can tell the length of a string actually CALLERID(num) in the dialplan. How is that done? If need to test the length of the CALLERID(num) if its less the 10 digits I need to set it to a known value or insert 0's at the beginning until it is 10 digits in length. My PRI provider needs it set to 10 digits always. Thanks, Jerry