.CTT File Extension
What is a CTT file?
Stores a list of contacts and groups used by the MSN/Live Messenger instant messaging program; saved as a text file and formatted in XML; can be exported from MSN Messenger and imported by another user.
Contacts saved in a CTT file can be imported into Messenger by selecting File→Import Contacts from a Saved File.
NOTE: Windows Live Messenger was discontinued in 2013 and has moved to Skype.