Guatam,
CDR retention periods are designated by the industry you are in. Government agencies will be much, much longer than regular companies and this will vary by Country and Company as well. Which agency is this phone system installed for? The DoT in the USA is the department of transportation but is usually written as DOT. Since you wrote DoT, it might be Department of Technology, but I don't know. If you are more specific there are plenty of people from many different countries that would be able to either answer, or point you in the right direction.
Jason
