Discreet SSR Codes are allocated to a country by ICAO. Where possible international flights are kept on the same code from departure point to destination.
Another group of codes are available for domestic use. These are managed by Air Services allocation to the ATS units and military. The TAAATS system will allocate codes as they are needed and become available.
It is quite normal for ATC to change your code back to 1200 exiting CTA as it is the code allocated to VFR aircraft in a particular airspace.