Can you post the dial plan configured in the ATA ? That's the only place where it could add digits automatically in front of the number. The syntax for pre-pending / replacing digits is something like
(<123:456>[2-9]xxxxxx)
which means "if a number matches 123[2-9]xxxxxx it will be converted to 456[2-9]xxxxxx".
In your case, you may have something like
(<:1>[2-9]xxxxxxxxx)
which means "prepend 1 to any 10 digit number startting with 2...9" (I usually use this for vbuzzer ATAs which require 1 in front of all numbers, including local).